THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


LECTURE  NOTES  ON 
PHYSIOLOGY 


BY 
HENRY  H.  JANEWAY,  M.D. 


THE  NERVOUS  SYSTEM 


NEW  YORK 

PAUL    B.   HOEBER 
67-69  EAST  SQTH  STREET 


Copyright,  1915, 
BY  PAUL  B.  HOEBEK 

Reprinted  October,  1917,  and  0          ,  1918 


M371676 


I 
THE  NERVES 

THE  PERIPHERAL  NERVES 

STRUCTURAL   BASIS   OF   THE   NERVOUS   SYSTEM 

The  Purpose  Served  by  the  Nervous  System  —  The  nervous 
system  has  developed  in  order  that  a  rapid  communication  be- 
tween the  distant  portions  Of  the  body  may  be  possible.  Its  tis- 
sues in  the  process  of  specialization  of  function  have  acquired  the 
highest  perfection  of  the  vital  phenomena  of  excitability  and  of  the 
power  of  transmission  of  a  change  dependent  on  excitement. 
Among  unicellular  animals  special  provision  for  such  a  means  of 
communication  does  not  exist.  Among  the  metazoa,  i.e.,  the  sponges, 
no  evidence  of  a  nervous  system  exists.  It  is  in  the  Coelenterata 

that  the  first  evidences  of  a  nervous  system  are  met  with. 

i 

DEVELOPMENT   OF   THE   NERVOUS   SYSTEM 

The  Hydra  —  In  the  hydra  some  of  the  epithelial  cells  have  pro- 
longations which  join  or,  at  least,  come  into  contact  with  deeper 
cells  possessing  special  contractile  power.  (Fig.  1.)  We  can  imag- 
ine that  these  epithelial  cells  with  their  prolongations  have  become 
endowed  with  a  special  sensitiveness  to  external  irritants,  and  pos- 
sess the  power  of  quickly  transmitting  the  effects  of  the  external 
changes  upon  it  to  the  contractile  cells  and  in  a  manner  to  cause  the 
latter  to  respond  immediately. 

Ccelenterates  —  The  jelly  fish  presents  quite  an  advance  over  this 
simple  nervous  system  and  no  intermediate  stages  are  known. 
(Fig.  2.)  The  nervous  system  of  the  jelly  fish  is  limited  to  the 
region  beneath  the  margin  of  the  umbrella.  From  the  epithelium 
of  the  surface,  fibers  pass  inward  forming  a  network  around  the 
margins  of  the  umbrella.  In  this  network  there  are  thickenings 
in  which  are  situated  nerve  cells.  (Fig.  3.)  A  finer  network  of 

4 


THE  NERVOUS  SYSTEM 


Fig.  1. — Diagrammatic  illustration  of  the  evolution  of  the  reflex  arc. 
A  shows  a  single  cell  differentiated  into  a  conductive  (1),  and  contractile 
portion  (2).  In  B  the  conductive  portion  (1)  and  the  contractile  portion  (2) 
exist  a$  separate  cells  and  maintain  their  connection  with  the  sensory  element 
by  a  slender  conductive  portion  in  this  cell,  which  represents  a  nerve  (3). 
In  C  the  sensory  cell  (1)  and  the  contractile  element  are  separate  cells,  but 
the  connection  between  the  two  is  maintained  by  an  interpolation  of  a  new 
nerve  cell  receiving  an  afferent  extension  (3)  from  the  sensory  cells  and 
giving  off  an  efferent  extension  (5)  of  the  muscle  cell. 

fibers  originates  in  the  network  just  described  and  terminates 
around  muscular  cells  (cells,  in  other  words,  which  have  acquired  in 
the  process  of  specialization  the  highest  perfection  for  that  in- 
dividual animal  of  the  vital  phenomenon  of  contraction).  Be- 
sides these  two  sets  of  nerve  fibers,  another  set  of  fibers  containing 
small  collections  of  cells  also  exists  beneath  the  Tnargins  of  the 
umbrella. 


THE  NERVOUS  SYSTEM 

The  various  sensitive  cells  on  the  surface  present  differences  in 
their  capabilities  of  responding  to  various  stimuli.  Such  differ- 
ences represent  specialization  of  the  function  of  excitability.  Some 
are  more  sensitive  to  light,  others  to  the  weight  of  a  crystal  of  lime 
developed  near  them,  and  still  others  to  chemical  and  contact  stim- 


•TENTACL.ES 


Fig.  2. — Diagram  of  a  jelly  fish. 
In  this  organism  the  central  nervous  cells  are  peripherally  placed. 


uli.  By  cutting  off  the  marginal  ring  with  its  marginal  bodies  we 
will  remove  the  special  sense  organs  and  the  ganglion  cells  of  the 
umbrella.  Such  a  mutilated  jelly  fish  lies  perfectly  motionless  in 
the  water.  It  is  incapable  of  any  automatic  activity  because  de- 
prived of  cells  sensitive  to  external  changes  its  muscle  cells  receive 
no  stimuli.  If  a  stimulus  is  applied  to  the  cut  nerve's  running  to 

8 


THE  NERVOUS  SYSTEM 


the  contractile  cells  within,  the  jelly  fish  will  contract.     Under 

these  conditions  the  manubrium  will  bend  in  the  direction  of  the 

stimulus. 

A  more  Advanced  Stage  with  Centrally  placed  Ganglion  Cells 
-  In  the  jelly  fish  the  ganglion  cells,  which  we  may  term  perhaps 

switch  stations  or  relay  stations,  are  sit- 
-  uated  around  the  periphery  of  the  body. 
It  will  be  a  manifest  advantage  to  an 
animal  to  have  these  switch  stations  sit- 
uated centrally.  Animals  with  cen- 
trally situated  stations,  such  as  the 
worms,  represent  the  next  stage  in  the  de- 
velopment of  a  nervous  system.  (Fig.  4.) 
The  Crayfish  —  A  still  further  ad- 
vance is  represented  in  animals  such  as 
the  crayfish,  in  which  the  head  ganglia, 
those  in  the  direction  in  which  the  ani- 
mal moves  forward,  show  a  special  de- 
velopment. 

In  these  animals  we  have  the  rudi- 
ments of  projicient  sense  organs,  organs 
furnishing  the  animal  with  information 
of  what  is  in  the  course  of  its  advance. 
They  may  be  not  improperly  termed  or- 
gans of  foresight.  Through  the  con- 
necting strands  of  fibers  between  these 
anterior  organs  and  the  ganglia  behind 
them  impulses  may  be  sent  to  check  for- 
ward movement  when  danger  ahead  is 
scented.  These  impulses  are  the  begin- 
nings of  inhibitory  impulses.  In  all 
these  primitive  forms  of  nervous  sys- 
tems, as  in  the  jelly  fish,  the  nervous 

system  starts  its  development  from  the  surface  epithelial  cells. 
The  Sensory  Cell  —  The  differentiated  peripheral  sensory  cell 

possesses  two  processes  —  a  short  one  passing  to  the  surface,  and  a 

long  one  passing  back  to  intermingle  with  a  network  of  fibers  in  the 

interior  of  the  animal.     (Fig.  5.) 

The  Central  Ganglion  —  This  network  also  contains  ganglion 

10 


Fig.  3.— Illustrating  the  com- 
munications between  the 
muscle  (1)  on  one  side  of 
the  umbrella  and  the  sen- 
sory epithelium  (2)  upon 
the  other  side  through  the 
peripherally  placed  cells 
(3). 


THE  NERVOUS  SYSTEM 


Fig.  4. — Illustrating  the  stages  in  the  evolution  of  a  centrally  placed  nerve  cell. 

In  1  direct  communication  between  the  muscle  and  sensory  cell. 

In  2  indirect  communication  between  the  sensory  cell  and  the  muscle 
through  a  peripherally  placed  nerve  cell. 

In  3  indirect  communication  between  the  sensory  cell  and  the  muscle 
through  a  centrally  placed  nerve  cell. 

cells,  the  processes  of  which  also  intermingle  with  terminal  divisions 
of  the  processes  of  the  differentiated  surface  epithelial  cells.  The 
intermingling  of  the  fibers  forms  a  network  embedded  in  a  granular 
substance  more  or  less  encapsulated  and  forming  a  ganglion. 
While  many  of  the  fibers  of  the  ganglion  cells  participate  in  the 


Lumbricus 


Nereis 


Vertebrate 


Fig.  5. — Diagrams  showing  the  relative  position  of  the  sensory  cell  in  lum- 
bricus,  nereis,  and  vertebrata.     (Quain.) 

12 


THE  NERVOUS  SYSTEM 

formation  of  the  network,  one  long  process  from  some  of  the  gan- 
glion cells  passes  to  a  muscle  cell  of  the  animal.  It  is  believed  that 
some  divisions  of  the  long  fibers  from  the  differentiated  sensitive 
epithelial  cell  may  become  a  part  of  the  central  ganglion  cell  and 
pass  directly  through  it.  If  this  occurs  it  is  exceptional,  but  it  is 
significant  that  some  fibers  of  the  terminal  network  from  the  differ- 
entiated sensitive  epithelial  cell  may  pass  directly  into  a  fiber  run- 
ning to  a  muscle  without  at  any  time  becoming  a  part  of  a  central 


Fig.  6. — Transverse  section  of  a  human  embryo  of  24  mm.     (Quain.) 
ent,  entoderm  of  yolk-sac;  the  lines  indicate  the  points  of  the  splanchno- 
pleuric  layers  which  will  come  together  to  cut  off  the  gut  from  the  cavity  of 
the  yolk-sac;  my,  outer  wall  of  mesodermic  segment;  me,  the  part  of  its  wall 
which  forms  the  muscle-plate;  sc,  sclerotome;  coe,  ccelom. 

nerve  cell.  This  primitive  system  is  thus  composed  of  two  ele- 
ments —  the  receiving  element,  which  is  the  differentiated  sensitive 
epithelial  cell  with  its  short  and  long  process,  and  the  reactive  ele- 
ment, or  the  peripherally  running  nerve  to  the  muscle,  which  may 
or  may  not  arise  in  a  central  nerve  cell.  The  one  is  called  the  sen- 
sory or  afferent  neuron  and  the  other  the  motor  or  efferent  neuron. 

The  Embryological  Development  of  the  Nervous  System  of  the 
Vertebrates  —  The  nervous  system  of  vertebrates  is  developed  from 
the  epithelium  of  a  groove  which  forms  upon  the  dorsum  of  the 
embryo.  This  groove  -subsequently  becomes  transformed  into  a 
canal.  At  the  front  end  three  cavities  become  formed  from  which 
the  three  brains  develop.  From  the  greater  length  of  the  canal 
posteriorly  the  spinal  cord  forms.  (Figs.  6-9.) 

The  Spongioblasts  and  Neuroblasts  — 'The  canal  is  formed  of 

14 


THE  NERVOUS  SYSTEM 

columnar  cells  between  the  outer  ends  of  which  small  rounded  cells 
are  found.  From  the  columnar  cells,  called  spongioblasts,  is  formed 
the  neuroglia  by  the  production  of  branching  processes.  Many  of 
the  columnar  cells  wander  externally  and  become  transformed  into 
round  cells  with  many  branches.  These  branches  form  the  sup- 
porting network  of  the  nervous  system,  the  neuroglia.  (Figs.  10- 


Yolk-sac. 


Amnion. 


Neural     groove. 


Neurenteric  canal. '  ~ 

Primitive  streak. 
Abdominal  stalk. 


Fig.  7. — Surface  view  of  early  human  embryo,  2  mm.  in  length  (after  Graf.  v. 

Spec.)    x  30  diameters.      (Quain.) 

The   amnion   is   opened,  and    on  the   blastoderm   are   seen   the   primitive 
streak,  the  dorsal  opening  of  the  neurenteric  canal,  and  the  neural  groove. 


13.)     The  round  cells  appearing  in  the  intervals  between  the  outer 
ends  of  the  columnar  cells 'are  termed  neuroblasts. 

The  Development  of  the  Sensory  and  Motor  Nerve  Fibers  - 
From  the  neuroblasts  grow  out  a  process  which  at  first  has  a  bulb- 
shaped  extremity.  (Fig.  15.)  By  continued  growth  of  the  process 
finally  reaches  the  periphery,  to  end  in  a  muscle  or  gland.  (Figs. 
14,  15.)  This  process  is  called  the  axis  cylinder  of  the  nerve  cell. 
After  the  growth  of  the  axis  cylinder  is  well  advanced  other  proc- 
esses grow  out  from  the  cell  and  terminate  ultimately  in  a  series  of 

16 


THE  NERVOUS  SYSTEM 

branches  called  dendrites.  These  cells  constitute  the  efferent  path 
of  the  central  nervous  system.  The  afferent  path  develops  from 
cells  formed  outside  the  primitive  neural  groove  from  cells  which 


n.  f.    n.  gr.    n.  f. 


mes.1 


n.  f. 


Ill 


Fig.  8. — Transverse  sections  of  the  human  embryo  of  2  mm.  represented  in 

Fig.  7.     (Quain.) 

In  I,  which  is  most  anterior,  the  fore-gut  is  separated  off  from  the  yolk-sac. 
n.  gr.,  neural  groove;  n. /.,  neural  folds;  n.  pi.  (in  III),  neural  plate;  mes.1, 
intra-embryonic  mesoderm;  p.,  pericardial  crelom;  am.ect.,  amniotic  ecto- 
derm; mes.2,  amniotic  mesoderm;  ent.,  entoderm  of  yolk-sac;  mes?,  meso- 
derm of  yolk-sac;  not.pl.  (in  III),  notochord-plate. 


form  a  longitudinal  thickening  just  external  to  the  latter.  From 
these  cells  two  processes  grow  out,  one  from  each  pole.  (Figs.  17- 
20.)  The  peripheral  one  grows  to  the  surface  to  terminate  in  a 
sentient  epithelial  cell.  The  central  one  grows  internally  into  the 

18 


THE  NERVOUS  SYSTEM 


Fig.  9. — Closure  of  neural  canal  of  human  embryo,  showing  the  cells  of  the 
neural  crest  becoming  separated  to  form  the  germs 

of  the  spinal  ganglia.     (Quain.) 
A,  canal  still  open;  B,  canal  closed. 


Fig.  10. — Neuroglia  cells  and  fibres  from  the  white  matter  of  the  human 
cerebellum  stained  by  Weigert's  neuroglia  stain.  A,  Neuroglia  cell;  B, 
blood-vessel  cut  longitudinally,  and  C,  blood-vessel  cut  transversely,  show- 
ing enveloping  neuroglia  fibres;  a,  neuroglia  fibres;  b,  cytoplasm  of  neu- 
roglia cell.  (Bailey.) 

20 


THE  NERVOUS  SYSTEM 


Fig.   11. — A,    Neuroglia   cell — spider   type — human   cerebrum.     B,   Neuroglia 
cell — mossy  type — human  cerebrum.     (Bailey.) 


Fig.  12. — Neuroglia-cells  of  cerebellum.     Golgi  method.     (Quain.) 
a,  spider-cells;   b,  arborescent  cells;   c,  ependyma-like  cells. 

22 


THE  NERVOUS  SYSTEM 


Fig.  13. — A  neuroglia-cell,  isolated  in  33  per  cent,  alcohol.     (Quain 


Fig.  14. — A,  ventral  root-fibres;  B,  dorsal  root-fibres;  C,  a  neuroblast  be- 
ginning to  bud  out;  D,  a  neuroblast  with  long  fibre  passing  towards  ven- 
tral commissure;  E,  a  motor  neuroblast  with  axon  and  dendrons;  F,  a 
motor  neuroblast  with  axon  only:  the  axon  is  expanded  at  the  growing 
end;  a,  a,  neuroblasts  with  axons  growing  into  the  lateral  column;  c,  grow- 
ing end  of  axon  of  a  commissural  fibre;  d,  a  cell  of  the  spinal  ganglion. 
(Quain.) 


24 


THE  NERVOUS  SYSTEM 


A 


Fig.   15. — Neuroblasts  from   the   spinal   cord   of  a   third-day   chick-embryo. 

(Quain.) 

A,  three  neuroblasts,  stained  by  Cajal's  reduced-silver  method,  showing  a 
network  of  neurofibrils  in  the  cell-body;  a,  a  bipolar  cell.  B,  a  neuroblast 
stained  by  the  method  of  Golgi  showing  the  incremental  cone  (c). 


a  be 

Fig.  16. — Section  of  wall  of  neural  tube  (first  cerebral  vesicle)   of  chick  of 

three  and  a  half  days.     (Quain.) 

A,  germinal  layer  containing  rounded  neuroblasts,  a,  b,  c  (these  already 
possess  fibrils);  B,  bipolar  neuroblasts;  c,  enlarged  growing  end  of  axon; 
e,  an  axon  growing  tangentially. 


26 


THE  NERVOUS  SYSTEM 


Fig.  17-A. — Chick-embryo  of  the  fifth  day.    (Quain.) 

A,  ventral  root;  B,  dorsal  root;  C,  motor  nerve-cells;  D,  sympathetic 
ganglion-cells;  E,  spinal  ganglion-cells  still  bipolar;  F,  mixed  nerve;  b,  c.  d, 
motor  nerve-fibres  passing  to  and  ramifying  in  /,  developing  dorsal  muscles; 
e,  a  sensory  nerve-trunk. 


Fig.  17-B. — Spinal  ganglion-cells  showing  transition  from  bipolar  to  unipolar 

condition.     (Quain.) 


28 


THE  NERVOUS  SYSTEM 

spinal  cord  to  terminate  in  the  neighborhood  of  some  central  cell 
developed  from  the  original  neural  groove.  The  two  processes  of 
the  afferent  cell  at  their  origin  from^the  cell  ultimately  approach 


Fig.  18. — Diagram  of  the  arrange- 
ment of  the  sensory  nerve-fibres 
in  the  olfactory  organ  and  bulb. 
(Quain.) 

n,  nerve-fibre  coming  off  from 
sensory  nerve  cell;  gl.,  synapse 
within  olfactory  glomerulus;  n, 
nerve-cell  and  nerve  of  olfactory 
bulb  of  brain. 


Fig.  19. — Diagram  of  the  connec- 
tions of  the  retinal  elements. 
(Quain.) 

s,  sensory  nerve-cells ;  gr.  i,  in- 
ner granules;  ra. i.,  inner  molec- 
ular layer;  g.,  ganglion-cell;  n, 
its  nerve-fibre  process  ramifying 
in  the  nerve-centre. 


each  other  so  that  they  finally  form  a  T  and  appear  to  be  given  off 
from  a  common  stem.  Because  of  the  double  process  originally 
possessed  by  these  cells  they  are  called  in  the  early  period  of  their 
development  bipolar  cells.  (Figs.  17,  A  and  B.)  In  mammals  all 
ultimately  become  unipolar  except  the  cells  of  the  spiral  and  ves- 
tibular  ganglia,  from  which  the  fibers  of  the  eighth  cranial  nerve 
grow.  These  retain  the  primitive  bipolar  arrangement. 


30 


THE  NERVOUS  SYSTEM 

The  Development  of  the  Medullary  Sheath  —  Some  time  after 
the  outgrowth  'of  the  axis  cylinder  the  medullary  sheath  is  formed, 
apparently  through  the  agency  of  the  axon  itself.  The  philogenet- 


auditory 


gustatory 


tactile 


Fig.  20. — Diagram  showing  the  mode  of  termination  of  sensory  nerve-fibres 
in  the  auditory,  gustatory,  and  tactile  structures  of  Vertebrata.     (Quain.) 

ically  youngest  fibers  in  the  body  acquire  a  medullary  sheath  later 
than  others.  Representatives  of  this  class  are  the  fibers  of  the 
pyramidal  tracts  and  the  long  posterior  columns  of  the  spinal  cord. 


THE  NERVOUS  SYSTEM 

THE   MORPHOLOGY   OP   NERVOUS   TISSUE 

The  Structure  of  a  Nerve  Cell  (Figs.  21-30)  — A  nerve  cell 
possesses,  like  all  cells,  a  nucleus.  The  nucleus,  though  of  large 
size,  contains  very  little  chromatin,  generally  collected  as  two  small 
nucleoli  within  the  nucleus.  Throughout  the  nerve  cell  run  many 
fibrillge  which  appear  in  well  prepared  specimens  as  delicate  stria- 
tions.  These  fibrillae  are  continued  out  of  the  cell  into  the  processes 
of  the  cells  between  the  Nissl  substance.  The 
Nissl  substance  is  very  abundant  except  at  that 
region  from  which  the  axis  cylinder  leaves  the 
cell.  In  this  region  many  fibrillae  are  collected 
together  to  enter  the  axis  cylinder.  It  is  called 
the  axon  hillock  of  the  cell.  The  cell  processes 
are  of  two  kinds  and  have  already  been  indi- 
cated. 

The    Axis   Cylinder   and   Dendrites  —  The 
axis  cylinder  is  smaller  than  the  other  processes, 
where  it  leaves  the  cell,  but  much  longer,  run- 
Fig.  21.— Two  mo-  ning,  in  the  case  of  motor  cells  of  the  spinal 
tor     nerve  -  cells  . ,  .    ,  «,-,-, 

from    the     dog.  cord>  to  tne  periphery  of  the  body. 
(Quain.)  The  dendrites  are  usually  thick  where  they 

after   a°"eriod   of   leave    the    Cel1   ^ut    SOOn    break   UP    into   manv 
prolonged      activ-  processes  which  form  a  network  with  similar 

ffom<Pp?epSo^  Processes  of  other  cells  not  far  from  the  cell 
by     Dr.     Gustav  from  which  they  originate. 

Neurons  —  A  nerve  cell  with  its  processes  is 

termed  a  neuron.    The  function  of  neurons  is  to 

transmit  nervous  impulses  and,  corresponding  to  the  direction  of 

the  impulse,  there  are  two  varieties  of  neurons  —  the  sensory  or 

afferent  neurons,  and  the  motor  or  efferent  neurons. 

An  afferent  neuron  functionally  connected  through  the  central 
nervous  system  with  an  efferent  neuron  constitutes  a  reflex  arc. 
Though  an  impulse  may  be  transmitted  in  either  direction  along  a 
nerve  fiber  it  can  only  traverse  a  reflex  arc  in  one  direction.  This 
phenomenon  is  termed  the  law  of  forward  direction. 

The  Structure  of  Nerves  (Figs.  31-32)  —All  the  nerves  which 
are  given  off  from  the  central  nervous  system,  the  brain  and  spinal 
cord,  possess  a  medullary  sheath.  This  consists  of  a  fatty  substance 
termed  myelin,  imbedded  between  the  meshes  of  a  network.  The 

34 


THE  NERVOUS  SYSTEM 


Fig.     22- A. — Ramified     nerve-cell 
from  ventral  horn  of  spinal 

cord  of  man.     (Quain.) 
a,  axis-cylinder  process;  b,  cell- 
body  with  nucleus  and  clump  of 
pigment-granules;  d,  d,  dendrons. 


Fig.    22-B. — Axis-cylinder    process 

of  a  nerve  cell.  (Quain.) 
x,  x,  portion  of  nerve-cell  from 
spinal  cord  of  ox,  with  axis-cylin- 
der process,  a,  coming  off  from  it 
and  acquiring  at  a1  a  medullary 
sheath,  highly  magnified. 


36 


THE  NERVOUS  SYSTEM 


Fig.  23. — Multipolar  and  unipolar  types  of  nerve-cell.     (Quain.) 

A,  large  pyramidal  cell  of  cerebral  cortex,  human,  Nissl  method.     (Cajal.) 
a,  axon;  b,  cell-body;  c,  apical  dendron;  d,  placed  between  two  of  the  basal 
dendrons,  points  to  the  nucleus  of  a  neuroglia-cell. 

B,  unipolar  cell  from  spinal  ganglion  of  rabbit,  Nissl  method.     (Cajal.) 
a,  axon;    b,  circumnuclear  zone,  poor  in  granules;    c,  capsule;    d,  network 
within  nucleus;  e,  nucleolus. 


38 


THE  NERVOUS  SYSTEM 


Fig.   24. — Nerve-cells    of  kitten  '  (from   the   anterior   corpora   quadrigemina) 

showing  neuro-fibrils. 
a,  axon;  b,  c,  d,  various  parts  of  the  intracellular  plexus  of  fibrils. 


40 


THE  NERVOUS  SYSTEM 


W 


Fig.  25. — Nerve-cells  of  lizard:  A  and  D  during  activity,  B  and  C  during 

hibernation.     (Quain.) 

a  (in  J3),  axon;  b,  b  (in  A  and  B),  knob-like  endings  of  extraneous  fibrils; 
c,  d  (in  C),  superficial  and  deep  fibril  networks. 


42 


THE  NERVOUS  SYSTEM 


Fig.  26.— A  long-axoned  nerve-cell  from  the  cerebral  cortex.     (Quain.) 
a,  axis-cylinder  process  with  collaterals;  d,  d,  dendrons;  b,  body  of  cell. 


44 


THE  NERVOUS  SYSTEM 


Fig.  27. — A  short-axoned  cell  from  the  cerebral  cortex.  Golgi  method.   (Quain.) 
a,  a',  a",  axon  and  its  ramification;  d,  d,  d,  dendrons. 


46 


THE  NERVOUS  SYSTEM 


Fig.  28. — Cell  from  cerebral  cortex,  showing  varicosities  on  its  dendrons  and 

not  spines.     Methylene-blue  method.     (Quain.) 

a,  axon;  b,  c,  a  branching  collateral  (both  this  and  the  main  axon  show  a 
medullary  sheath) ;  d,  varicose  dendrons. 


48 


THE  NERVOUS  SYSTEM 


Fig.   29. — Synaptic  ramifications   of  axon   of  one   nerve-cell,  B,  around   the 

bodies  of  other  cells,  A.    From  the  cerebellum  of  the  rat.     (Quain.) 

a,  b,  c,  ramifying  axon;  d,  dendrons. 


Fig.  30. — Two  motor  cells  from  the  rabbit's  spinal  cord,  which  show  chromat- 
olysis  as  the  result  of  section  (fifteen  days  previously)  of  the 

nerve-fibres  which  arise  from  them.     (Quain.) 

In  A  the  chromatolysis  is  rather  less  advanced  than  in  B.  In  both  the 
nucleus  has  moved  to  the  periphery,  and  the  cell-substance  (b  and  c)  is 
swollen,  a,  axon-process  of  A. 


Fig.  31. — Transverse  and  longitudinal  section  of  medullated  nerve-fibre   of 

frog   (osmic  acid  and  acid  fuchsine).     (Quain.) 

The  longitudinal  section  shows  one  node  of  Ranvier  and  two  of  Lanter- 
mann's  clefts.  The  fibrillar  structure  of  the  axis-cylinder  is  shown  in  both 
longitudinal  and  transverse  section. 

50 


THE  NERVOUS  SYSTEM 


sz  ... 


network  is  composed  of  a  substance  termed  neurokeratin.    Outside 

the  myelin  substance  is  a  sheath 
termed  the  neurolemma.  Inside  the 
myelin  and  separating  it  from  the  axis 
cylinder  is  the  axolemma.  The  axis 
cylinder  itself  is  composed  of  many 
fine  fibrillge,  the  so-called  neurofibrillae. 
At  certain  intervals  along  the  nerve 
fiber,  intervals  proportionate  to  the 
diameter  of  the  nerve  fiber  and  vary- 
ing from  80  to  600  microns,  the  mye- 
lin substance  suffers  interruption  so 
that  the  medullary  sheath  dips  down 
to  touch  the  axis  cylinder.  These  in- 
terruptions are  called  the  nodes  of 
Ranvier.  The  nerve  fibers  of  the 
peripheral  nerves  run  in  bundles,  sur- 
rounded and  held  together  by  a  con- 
nective tissue  sheath  termed  the  peri- 
neurium. 


sz 


Fig.  32. — Scheme  of  structure  of  medullated 

peripheral  nerve  fibre  of  a  fish. 
-  ieo  (Nemileff.) 

A,  Cross  section;  B,  longitudinal  section; 
on  left  fibre  is  shown  as  stained  intra  vitam 
with  methylene  blue;  on  right,  myelin  is 
shown  black  as  in  osmic*  acid  staining  with 
the  incisures  of  Schmidt  indicated;  sz,  cells 
of  sheath  of  Schwann;  n,  their  nuclei;  ss, 
sheath  of  Schwann;  sp,  processes  of  the 
cells  of  sheath  of  Schwann  or  the  myelin 
sheath  network;  le,  larger  trabeculse  of  pro- 
-  sp  toplasmic  framework  of  medullary  sheath 
arranged  obliquely  to  axis-cylinder  and 
Pf  forming  the  so-called  "funnels";  Ieo,  clear 
streaks  in  fibres  treated  with  osmic  acid,  cor- 
responding to  le,  incisures  of  Schmidt;  mo, 
myelin  blackened  with  osmic  acid;  ax  axis- 
cylinder;  pa,  periaxial  space  around  axis- 
cylinder;  gs,  "coagulated  fluid"  in  periaxial 
space;  pf,  peripheral,  non-fibrillar,  part  of 
axis-cylinder;  f,  neurofibrils  of  axis-cylinder; 
r,  ring-like  thickening  of  Schwann's  sheath 
at  node  of  Ranvier;  o,  cavity  in  r.  (Bailey.) 

52 


THE  NERVOUS  SYSTEM 

The  Varieties  of  Peripheral  Endings  of  Sensory  Nerves  —  The 

peripheral  process  of  the  afferent  nerve  cell  forms  a  connection 
with  the  peripheral  epithelium  in  a  variety  of  ways. 


Fig.  33. — Nerve  and  nerve  endings  in  the  skin  and  hair  follicles.      (After 

G.  Retzius.) 

As,  Outer  root  sheath;  c,  most  superficial  nerve-fibre  plexus  in  the  cutis; 
dr,  sebaceous  glands;  h,  the  hair  itself;  hst,  stratum  corneum;  is,  inner  root 
sheath  of  hair;  n,  cutaneous  nerve;  rm,  stratum  germinativum  Malpighii. 
(Bailey.) 


1.  In  some  instances  it  merely  terminates  by  ramifying  between 
epithelial  cells.    Losing  its  medullary  sheath  it  divides  a  number 

54 


THE  NERVOUS  SYSTEM 


of  times.    Each  division  ending  in  a  little  knob  between  epithelial 
cells.     (Fig.  33.)     Nerve  fibers  end  in  this  manner  which  run : 

(1)  To  the  skin. 

(2)  To  the  mucous  membranes. 

(3)  To  connective  tissue. 

(4)  To  the  glandular  epithelium. 


(5) 
(6) 
(7) 


To  certain  serous  surfaces. 

To  the  outer  sheath  of  the  root  of  a  hair  follicle. 

To  the  teeth.    Whether  the  dentine  is  penetrated  by  nerve 
fibers  is  a  matter  of  dispute. 

2.  In  certain  regions  of  the  body  where  special  sensitiveness  is 
required  these  endings  are  formed  by  a 
combination  of  complicated  variations 
of  the  sensory  epithelium  with  the 
nerve  ending.  The  simplest  of  these 
special  end  organs  is  the  tactile  cell.  It 
consists  of  an  epithelial  cell  with  a  pro- 
longed inner  extremity  which  comes 
into  contact  with  a  leaf-like  expansion 
of  the  end  of  the  nerve  fiber. 

3.  A   more   complex   ending    is   the 
compound  tactile  cell.    This  consists  of 
several  epithelial  cells  grouped  together 
and  in  contact  with  one  nerve  ending. 
Representatives  of  this  class  are  the  cor- 
puscles of  Grandry  and  Merkel's  cor- 
puscles.    (Figs.  34  and  36.) 

4.  End  bulb.    In  this  form  the  bul- 
bous end  of  the  axons  terminate  in  a 
special  granular   matter  inclosed   in  a 
capsule    of   flattened    connective   tissue 
like  cells.    They  occur  in  the  mouth  and 
conjunctiva.     (Fig.  35.) 

5.  Compound  end  bulbs  are  combinations  of  several  simple  end 
bulbs  containing  several  nerve  endings.     They  occur  in  the  nose, 
rectum,  peritoneum  tendon,  ligaments,  joints  and  in  the  trunks  of 
nerves  upon  the  glans  penis  and  clitoris.     (Fig.  37.) 

6.  Pacinian  bodies.    In  this  form  the  axis  cylinder  terminates  in 
a  rod  which  is  inclosed  in  alternate  concentric  layers  of  a  modified 

56 


Fig.  34. — Tactile  corpuscle 
within  a  papilla  of  the 
skin  of  the  hand,  stained 
with  chloride  of  gold. 
(Quain.) 

n,  two  nerve-fibres  passing 
to  the  corpuscle;  a,  a,  vari- 
cose ramifications  of  the 
axis-cylinders  within  the 
corpuscle. 


THE  NERVOUS  SYSTEM 
a          t  t 


Fig.  35. — Herbst  corpuscle  of  duck.     (Quain.)     x  380  diameters. 
n,  medullated  nerve-fibres;  a,  its  axis-cylinder,  terminating  in  an  enlarge- 
ment at  end  of  core;  c,  nuclei  of  cells  of  core;   t,  nuclei  of  cells  of  outer 
tunics;  t',  inner  tunics. 


Fig.  36. — Corpuscles  of  Grandry  from  the  duck's  tongue.     (Quain.) 
A,  composed  of  three  cells,  with  two  interposed  discs,  into  which  the  axis- 
cylinder  of  the  nerve,  n,  is  observed  to  pass;  in  B  there  is  but  one  tactile 
disc  enclosed  between  two  tactile  cells. 


Fig.  37. — A  medullated  fibre  terminating  in  several  end-bulbs  in  the  human 
peritoneum.     Lower  power.     Methylene-blue  preparation.     (Quain.) 

58 


THE  NERVOUS  SYSTEM 


Fig.   38. — Magnified   view   of   a   Pacinian   body    from   the    cat's   mesentery. 

(Quain.) 

n,  stalk  of  corpuscle  with  nerve-fibres,  enclosed  in  sheath  of  Henle,  passing 
to  the  corpuscle  n',  its  continuation  through  the  core,  m,  as  axis-cylinder  only; 
o,  its  terminal  arborization;  c,  d,  sections  of  epithelioid  cells  of  tunics,  often 
mistaken  for  the  tunics  themselves;  /,  channel  through  the  tunics  which  ex- 
pands into  the  core  of  the  corpuscle. 


60 


THE  NERVOUS  SYSTEM 


Fig.  39. — Nerve-endings  upon  the  intrafusal  muscle-fibres  of  a  muscle-spindle 

of  the  rabbit.     Moderately  magnified.     Methylene-blue 

preparation.     (Quain.) 

a,  large  medullated  fibre  coming  off  from  "spindle"  nerve  and  passing  to 
end  in  an  annulo-spiral  termination  on  and  between  the  intrafusal  fibres; 
b,  fine  medullated  fibres  coming  off  from  the  same  stem  and  dividing.  Its 
branches,  c,  pass  towards  the  ends  of  the  muscle-fibres  and  terminate  in  a 
number  of  small  localized  arborizations,  like  end-plates. 


Fig.  40. — Sensory  nerve  terminating  in  arborizations  around  the  ends  of 
muscle-fibres.     (Quain.) 


Fig.    41  .--An   annulo-spiral    ending    of    intrafusal    fibre.      Highly    magnified. 
Methylene-blue   preparation.      (Quain.) 

62 


THE  NERVOUS  SYSTEM 


Fig.  42. — Organ  of  Golgi  from  the  human  tendo  achillis.     Chloride  of  gold 

preparation.     (Quain.) 

m,  muscular  fibres;    t,  tendon-bundles;    G,   Golgi's  organ;    n,  two  nerve- 
fibres  passing  to  it. 


Fig.  43. — Ending  of  nerve-fibres  in  cardiac  muscle.     (Quain.) 


Fig.   44. — Motor   nerve-ending   in  the   abdominal    muscles   of   a   rat.     Gold 
preparation.     Magnified   170  diameters.      (Quain.) 

64 


THE  NERVOUS  SYSTEM 


Fig.  45. — Terminal   nerve-fibrils   in   an   alveolus   of  the   submaxillary   gland 

of  the  dog.     Chromate  of  silver  method.  ^  (Quain.) 
The  extension  of  the  lumen  into  the  crescents  of  Gianuzzi  is  also  shown. 


Fig.  46. — Motor  end-organ  of  a  lizard.    Gold  preparation.     (Quain.) 
n,  nerve-fibre  dividing  as  it  approaches  the  end-organ;  r,  ramification  of 
axis-cylinder  upon,  b,  granular  bed  or  sole  of  the  end-organ;  m,  clear  sub- 
stance surrounding  the  ramifications  of  the  axis-cylinder. 


66 


THE  NERVOUS  SYSTEM 


Fig.  47. — Nerve-ending  in  muscular  fibre  of  lizard  (lacerta  viridis).     (Quain.) 
A,  end-plate  seen  edgeways;  B,  from  the  surface;  s,  s,  sarcolemma;  pp,  ex- 
pansion of  axis-cylinder.     In  B  the  expansion  of  the  axis-cylinder  appears 
as  a  clear  network  branching  from  the  divisions  of  the  medullated  fibres. 


Fig.  48. — Ending  of  motor  nerves  in  rabbit's  muscle.    Reduced  silver  method. 

(Quain.) 

a,  axis-cylinder  of  entering  nerve;    b,  c,   parts   of   terminal   ramification 
showing  network  of  neuro-fibrils. 


68 


THE  NERVOUS  SYSTEM 

,  epithelium  containing  fluid  between  them.  These  occur  in  the 
palms  of  the  hands,  the  soles  of  the  feet,  in  the  parietal  peritoneum, 
the  mesentery,  mammary  gland,  tendons,  ligaments,  joints,  and 
penis,  clitoris  and  in  the  voluntary  muscles.  (Fig.  38.) 

7.  Besides  the  Pacinian  bodies  sensory  nerves  to  muscular  fibers 
terminate  in  expansions  which  form  (a)  annular,   (&)  spiral  and 
(c)    arborizing   expansions   around  the   muscular  fibers.      (Figs. 
39-41.) 

8.  Special  arborizations  upon  the  tendons  are  termed  organs  of 
Golgi.    (Fig.  42.) 

9.  All  motor  nerves  terminate  at  the  muscular  fibers  by  branch- 
ing in  a  mass  of  granular  substance  superficial  to  the  muscular  fiber 
but  beneath  the  sarcolemma.    This  ending  is  termed  the  motor  end 
plate.    All  these  modifications  of  endings  have  developed  for  the 
purpose  of  delicately  transmitting  slight  molecular  changes  of  a 
refined  order  to  or  from  a  sensitive  cell.     (Figs.  43-48.) 

The  more  complex  the  connection  between  the  termination  of  the 
nerve  and  the  sensitive  surface,  the  more  refined  and  delicate  and 
special  is  the  sensation  or  motor  impulse  transmitted.  In  other 
words,  in  those  regions  in  which  sensations  of  a  special  sensitive- 
ness or  of  a  special  kind  must  be  transmitted  there  is  need  of  a 
special  device,  which  exists  in  the  form  of  these  complex  nerve 
endings. 

The  importance  of  even  simple  nerve  endings  is  made  clear  by 
the  disorderly  character  of  a  reflex  excited  by  stimulating  the  cut 
nerve  endings  after  dissecting  off  the  skin. 

70 


THE  NERVOUS  SYSTEM 


PHYSIOLOGY   OF   NERVES 

The  Classification  of  Nerves  —  Nerves  may  be  classified  as  fol- 
lows i—- 


Efferent.  . 


'  Excitatory 


Motor 


Secretory  - 


Motor 

Vasomotor 

Cardiomotor 

Visceromotor 

Pilomotor 

Salivary 
Gastric 
Pancreatic 
Sweat 


Inhibitory 


Trophic 


Afferent . 


Excitatory  «« 


f  Motor  for  all  sub.  div.  of  motor  nerves 


[  Secretory  for  all  sub.  div.  of  secretory  nerves 


Visual 

Auditory 

Equilibrial 

Olfactory 

Gustatory 

Pressure 

Temperature 

Pain 

Hunger 

Thirst 


r  Sensory 


Reflex 


Reflex  for  various  efferent  nerves 


Inhibitory 


Trophic 


Inhibitory  upon  conscious  sensations  have  not 
been  demonstrated 

The  reflex  fibers  which  cause  unconscious  re- 
flexes are  known  to  be  inhibited  in  some 
cases  at  least 


72 


THE  NERVOUS  SYSTEM 

The  Velocity  of  an  Impulse  along  a  Motor  Nerve  is  measured  by 
stimulation  of  the  nerve  of  a  muscular  nerve  preparation  at  two 
points,  separated  by  a  known  distance,  and  recording  upon  a  mov- 
ing surface  the  time  of  application  of  each  stimulus  and  the  time 
of  response  (contraction  of  the  muscle)  to  each  stimulus. 

The  difference  between  the  time  of  application  of  the  stimulus 
and  the  response  in  the  two  cases  will  be  the  time  it  has  taken  the 
impulse  to  travel  the  known  distance  which  separates  the  electrodes. 
The  velocity  in  a  frog's  nerve  is  28  meters  a  second,  and  in  warm 
blooded  animals  is  60  to  120  meters  a  second.  (Fig.  49.) 


Fig.  49. — Apparatus  arranged  for  determining  speed  of  motor  nerve  impulse. 
By  means  of  turn-over  key  (1),  the  current  from  the  cell  (2),  through  the 
inductorium  (3)  may  be  applied  to  the  nerve  at  the  two  points  (4  and  5). 
The  difference  in  time  in  the  contraction  of  the  muscle  is  indicated  by  the 
difference  in  the  rise  of  the  lever  (6)  on  the  cylinder  (7)  in  fractions  marked 
at  (8). 

The  Velocity  of  an  Impulse  along  a  Sensory  Nerve  can  only  be 
measured  by  measuring,  in  the  same  manner,  the  velocity  of  the 
electrical  change  which  accompanies  the  propagation  of  the  im- 
pulse. It  is  about  the  same  as  the  velocity  of  propagation  along  a 
motor  nerve. 

The  velocity  of  propagation  varies  with  the  species  of  animal. 
In  general  it  is  proportional  to  the  height  in  the  scale  of  life  of  the 
animal  in  question. 

The  Direction  of  the  Impulses  —  The  direction  of  propagation 
along  a  nerve  fiber  may  be  ascertained  by  noting  the  direction  of 
spread  of  the  electrical  current  accompanying  the  propagation  of 
the  impulse.  The  impulse  is  found  to  travel  in  both  directions  from 
the  point  stimulated. 

74 


THE  NERVOUS  SYSTEM 

A  Demonstration  that  Nerve  Impulses  Travel  in  both  Direc- 
tions—  The  gracilis  muscle  of  a  frog  is  separated  longitudinally 
into  halves  hy  a  tendinous  intersection.  The  axis  cylinders  supply- 
ing these  two  divisions  are  divisions  of  the  axis  cylinders  them- 
selves, which  run  to  the  muscle.  Stimulation  of  the  distal  portion 
of  the  divisions  of  the  axis  cylinders  supplying  one  half  will  cause 
a  contraction  of  the  whole  muscle,  whereas  stimulation  of  the  muscle 
alone,  i.e.,  in  a  place  free  from  nerve  fibers,  will  cause  a  contraction 
in  only  one  half  of  the  muscle.  In  the  first  instance  the  impulse 
must  have  traveled  up  the  set  of  axis  cylinders  of  the  stimulated 
half  and  down  the  branches  into  the  unstimulated  half,  thus  travel- 
ing in  both  the  direction  of  the  course  of  the  nerve  in  one  part  and 
in  the  opposite  direction  to  the  course  of  the  nerve  in  the  other  part. 

Bell  and  Majendie 's  Law  —  Though  an  impulse  may  travel  in 
either  directibn  of  a  nerve,  the  same  nerve  cannot  be  both  afferent 
and  efferent.  This  fact  has  been  enunciated  into  a  law  by  Bell  and 
Majendie  and  is  known  as  their  law.  The  difference  in  the  function 
of  nerves  expressed  by  the  law  of  Bell  and  Majendie  is  not  depen- 
dent upon  any  essential  difference  in  the  nerve  but  solely  upon 
their  central  and  peripheral  connections,  a  fact  which  may  be  dem- 
onstrated by  grafting  experiments. 

Events  Accompanying  the  Passage  of  a  Nerve  Impulse  —  The 
Expenditure  of  Energy  —  The  fact  that  a  nerve  loses  its  irritability 
in  the  absence  of  oxygen  indicates  that  the  process  of  excitation  is 
accompanied  by  the  consumption  of  oxygen  and,  therefore,  the 
dissipation  of  energy.  The  consumption  of  energy,  however,  must 
be  extremely  small  inasmuch  as  the  most  sensitive  methods  for 
measuring  heat  have  failed  to  detect  any  rise  of  temperature  accom- 
panying the  passage  of  a  nervous  impulse. 

The  Demarcation  Current  —  If  the  terminals  of  a  delicate  gal- 
vanometer are  connected  to  a  resting  uninjured  nerve  no  current 
through  the  nerve  will  be  detected.  If,  however,  the  nerve  is  di- 
vided and  one  of  the  poles  of  the  galvanometer  circuit  is  connected 
with  the  injured  end,  while  the  other  pole  is  in  contact  with  the  side 
of  the  nerve  at  some  distant  point,  the  needle  of  the  galvanometer 
will  swing,  indicating  in  the  first  place  the  existence  of  a  current 
in  the  nerve  and  in  the  second  place  that  the  current  passes  through 
the  nerve  from  the  end  pole  to  the  lateral  pole.  In  terms  of  the 
outside  circuit  the  pole  on  the  end  of  the  nerve  is  negative  to  all 

76 


THE  NERVOUS  SYSTEM 

other  points.    This  current  is  called  a  demarcation  current  and  is 
excited  by  the  injury  to  the  nerve  incidental  to  its  division. 

Current  of  Action  —  If  now  an  impulse  is  excited  in  the  nerve 
by  stimulating  it  above  the  site  of  application  of  the  poles  of  the 
galvanometer  the  needle  of  the  galvanometer  will  swing  in  an  oppo- 
site direction,  indicating  a  current  in  the  opposite  direction.  This 
current  is  the  current  of  action  or  the  current  accompanying  the 
passage  of  an  impulse  which  passes  from  the  stimulated  point  in 
both  directions  and  consequently  toward  the  injured  end.  It  has 
been  called  the  negative  phase  of  the  demarcation  current.  The 
demarcation  current  is  strongest  immediately  after  division  of  the 
nerve  and  quickly  subsides  as  the  nerve  dies  up  to  the  node  of 
Ranvier  nearest  to  the  cut  end.  It  may  again  be  excited  by  a  fresh 
division  above  this  node.  The  current  of  action  travels  with  the 
same  rate  as  the  nervous  impulse  —  28-33  meters  a  second.  It  is 
18  m.  in  length  and  lasts  only  6/10,000-8/10,000  of  a  second  at  any 
one  point. 

Conditions  Affecting  the  Passage  of  a  Nervous  Impulse  — 
Temperature  —  Conduction  along  a  nerve  is  much  diminished  by 
decreasing  the  temperature.  A  temperature  between  0°  and  5°  C. 
is  sufficient  to  check  conductivity  in  a  mammalian  nerve.  The 
temperature  coefficient  for  each  difference  in  temperature  of  10°  C. 
is  1.79.  This  amount  is  a  constant  factor  by  which  the  velocity 
of  conduction  along  a  nerve  fiber  may  be  multiplied  to  give  the 
velocity  at  10°  C.  higher  temperature. 

Fatigue  —  Nerve  fibers  cannot  be  fatigued,  at  least  by  excessive 
excitation,  but  a  nerve  muscular  preparation  quickly  shows  signs 
of  fatigue  from  repeated  excitation. 

Demonstration  of  the  Site  of  Fatigue  in  the  Nerve  Muscle 
Mechanism  —  After  the  preparation  has  been  fatigued  the  muscle 
may  be  excited  to  contraction  by  direct  stimulation,  showing  that 
the  muscle,  at  least,  is  not  the  most  quickly  fatigued  of  the  various 
components  of  a  muscle  nerve  preparation.  In  considering  the  seat 
of  fatigue  of  a  nerve  muscular  preparation,  besides  the  muscle, 
the  motor  end  plate  and  the  nerve  are  to  be  considered.  Two 
agents  will  enable  us  to  eliminate  the  nerve  as  the  seat  of  fatigue. 
One  is  curare  and  the  second  is  the  passage  of  the  constant  cur- 
rent. Curare  specifically  paralyzes  the  motor  end  plate.  If  it  is 
applied  to  a  muscular  nerve  preparation  the  nerve  may  be  con- 

78 


THE  NERVOUS  SYSTEM 


tinuously  stimulated  without  affecting  the  motor  end  plate  or  the 
muscle,  because  of  the  block  produced  at  the  motor  end  plate  by 
curare.  After  the  effect  of  the  curare  wears  off  or  after  it  is  set 
aside  by  its  antagonist  physostigmin,  the 
muscle  again  may  be  stimulated  by  stimula- 
tion of  the  nerve,  showing  that,  although  the 
nerve  had  been  excited  for  a  much  longer 
time  than  would  have  been  necessary  to 
fatigue  both  the  motor  end  plate  and  the 
muscle  during  the  time  that  both  were  pro- 
tected by  the  curare,  the  nerve  is  still  capable 
of  transmitting  an  impulse. 

A  constant  current  possesses  the  power  of 
blocking  impulses  through  a  nerve.  It  may  be 
used  in  the  same  manner  as  curare  to  protect 
the  motor  end  plate  and  muscle  from  excita- 
tion during  a  period  of  prolonged  stimulation 
of  the  nerve  of  a  nerve  muscle  preparation 
above  the  point  of  application  of  the  constant 
current.  (Fig.  50.) 

Drugs  —  The  action  of  drugs  upon  nerves 
may  be  tested  by  inclosing  the  nerve  of  a 
muscle  nerve  preparation  within  a  closed 
tube.  The  nerve  issues  through  the  ends  of 
the  tube,  which  are  closed  with  normal  saline 
clay.  Into  the  tube  may  then  be  conducted 
the  vapor  of  the  drug,  the  action  of  which  it 
is  desired  to  test.  The  effect  of  these  drugs 
may  then  be  tested  upon  both  the  excitability 
of  the  nerve  and  its  conductivity. 

Excitability  may  be  tested  by  means  of 
electrodes  which  make  connections  with  the 
portion  of  the  nerve  inclosed  within  the  tube. 
Conductivity  may  be  tested  by  electrodes 
making  contact  with  a  portion  of  the  nerve 
outside  of  that  end  of  the  tube  which  is  opposite  to  the  end  nearest 
the  muscle  end  of  the  nerve  muscle  preparation. 


Fig.     50.  —  Indefatig- 
ableness  of  the 

nerves. 

Two  muscles,  Mi, 
Mz,  furnished  with 
their  nerves,  Ni,  Na, 
are  simultaneously 
stimulated  at  x  by  an 
induced  current.  The 
nerve,  Nz,  is  anelec- 
trotonized  at  B  by  a 
constant  current,  Z, 
so  as  to  prevent  the 
impulse  reaching  the 
muscle,  M2,  thus  to 
prevent  this  muscle 
being  fatigued.  The 
muscle,  Mi,  is  quick- 
ly fatigued  and  ceases 
to  contract.  If  then 
the  cell  current  is 
broken,  while  the 
stimulation  of  the 
two  nerves  continues 
at  x,  the  muscle,  M2, 
will  be  seen  to  con- 
tract ;  therefore  its 
nerve  has  not  felt  the 
effects  of  fatigue. 


80 


THE  NERVOUS  SYSTEM 


Fig.  51. — Apparatus  for  exposing  a  portion  of  a  nerve  to  gases  and  for  testing 
the  excitability  and  conductivity  of  the  exposed  portion 

1»  Exposed  portion  of  nerve. 

2.  Tube  containing  the  gas  with  inlet  and  outlet. 

3.  Electrodes  for  testing  excitability. 

4.  Electrodes  for  testing  conductivity. 


Effect  of  Carbon  Dioxide  and  Ether,  Chloroform  and  Alcohol  — 
Carbon  dioxide  and  ether  diminish  first  excitability,  and  then  con- 
ductivity. Chloroform  rapidly  diminishes  excitability  and  conduc- 
tivity and  far  more  intensely  than  ether,  so  that  recovery  may  not 
be  complete.  Alcohol  diminishes  conductivity  without  at  first 
affecting  excitability.  (Figs.  51,  52.) 


Fig.  52. — Illustrates  the  effect  of  ether  vapor  upon  excitability  and  con- 
ductivity. Following  the  exposure  of  the  nerve  to  ether  there  is  a  disap- 
pearance of  excitability.  The  dotted  vertical  lines  illustrate  a  response  to 
the  application  of  the  current  to  the  portion  of  the  nerve  surrounded  with 
ether  vapor.  The  straight  vertical  lines  indicate  a  response  of  the  nerve 
to  the  stimulation  applied  to  a  point  outside  the  portion  exposed  to  the 
ether  vapor.  The  stimulus  is  therefore  alternately  applied  to  the  nerve 
inside  and  outside  of  the  tube. 

At  1  there  is  a  disappearance  of  excitability. 

At  2  there  is  a  reappearance  of  excitability. 

At  3  there  is  a  disappearance  of  excitability. 

At  4  there  is  a  disappearance  of  both  excitability  and  conductivity. 

At  5  there  is  a  reappearance  of  conductivity  and 

At  6  a  reappearance  of  excitability. 

82 


THE  NERVOUS  SYSTEM 

EVENTS  ACCOMPANYING  THE   ELECTRICAL  EXCITATION   OP  NERVES 

The  Minimal  Effective  Stimulus  is  ascertained  by  exciting  a 
nerve  muscular  preparation  from  the  discharge  of  a  condenser 
charged  with  decreasing  potential  until  the  minimal  stimulus  is 
found.  (Fig.  53.)  For  frogs'  nerve  it  is  1/1000  of  an  erg  (an  erg 
being  the  amount  of  energy  produced  or  work  performed  by  the 
action  of  one  dyne  through  one  centimeter.  A  dyne  gives  an  ac- 
celeration of  one  centimeter  per  second  to  one  gram). 

Summations  —  If  several  subminimal  stimuli  are  applied  suf- 
ficiently close  together  so  that  each  successive  stimulus  affects 


Fig.  53. — Illustrating  the  apparatus  for  exciting  the  nerve  from  a  condenser 

with  a  definite   quantity   of  electricity. 

1.    Switch  for  throwing,  first,  the  rheostat,  2,  and,  second,  the  condenser,  3, 
into  the  circuit  of  the  electrodes  upon  the  nerve. 

the  nerve  before  the  effect  from  the  first  one  has  passed  off  the 
combined  effect  may  produce  excitation  though  each  individual 
stimulus  would  fail  to  do  so.  This  phenomenon  is  called  sum- 
mation. 

The  Refractory  Period  —  For  a  brief  time  after  the  application 
of  an  electrical  current  to  a  nerve  it  remains  unexcited.  This 
period  is  called  the  refractory  period  and  amounts  to  .002-. 0006 
of  a  second  according  to  the  temperature.  The  existence  of  a 
refractory  period  is  common  to  all  forms  of  excitable  tissue,  and 
is  best  illustrated  in  heart  muscle.  After  the  application  of  a 
stimulus  to  heart  muscle,  the  change  producing  contraction  may 
be  said  to  be  progressing,  and  it  is  during  this  period  that  the 
muscle  is  refractory  to  another  stimulus  of  the  same  strength,  for 
the  simple  reason  that  it  is  already  responding  to  the  first  stimu- 

84 


THE  NERVOUS  SYSTEM 

lus.  If,  however,  a  very  strong  stimulus  is  applied  within  the  re- 
fractory period  it  may  respond,  so  that  the  refractory  period  de- 
pends on  the  strength  of  the  stimulus  used.  However,  after  such  a 
response  it  remains  irresponsive  to  normal  stimuli  for  a  longer 
time,  thus  indicating  the  causes  upon  which  the  refractory  period 
depends,  namely  a  breaking  down  of  material  available  for  re- 
sponse, the  response  depending  upon  the  katabolic  changes. 

Site  of  the  Excitation  —  When  the  constant  current  is  made 
use  of  to  excite  a  nerve,  an  excitation  occurs  at  the  make,  and, 


Fig.  54. — Apparatus  for  determining  the  site   of  excitation  of  the  muscle, 
whether  at  the  anode  or  the  cathode,  at  either  the  make  or  the  break. 
B,  Battery;  K,  key,  by  means  of  which  the  current  is  made  or  broken; 

C,  clamp  holding  the  muscle  by  its  middle  movable  electrodes,  EE,  capable 

of  recording  the  movements  upon  the  paper,  P. 

assuming  that  the  current  is  strong  enough,  at  the  break  also;  a 
make  excitation  starts  at  the  cathode  and  a  break  excitation  starts 
at  the  anode.  Inasmuch  as  what  is  true  for  one  excitable  tissue  is 
true,  in  this  respect,  for  another,  muscle  may  be  used  to  demon- 
strate the  fact. 

^(a)  It  may  be  clamped  lightly  in  its  middle  and  the  two 
electrodes  attached  to  the  two  ends,  each  electrode  being  capable 
of  a  swing  in  towards  the  muscle.  At  the  make  contraction  the 
cathode  electrode  will  swing  in  towards  the  muscle;  at  the  break 
contraction  the  anode  will  do  the  same.  (Fig.  54.) 

(b)  Inasmuch  as  injury  or  death  of  the  end  of  a  muscle  will 
prevent  its  irritability,  a  muscle  injured  at  one  end  may  be  used 
to  demonstrate  the  starting  point  of  the  contraction  at  the  make 

86 


THE  NERVOUS  SYSTEM 

and  break.    At  the  make  no  contraction  will  occur  if  the  cathode 
is  attached  to  the  injured  end,  and  vice  versa. 

Electrotonus  —  It  has  been  said  that  contraction  only  occurs 
at  the  make  and  break  of  a  constant  current.  If  the  current  is 
strong  enough  the  excitability  of  the  nerve  of  a  muscular  nerve 
preparation  may  be  so  increased  that  the  muscle  may  be  thrown 
into  a  state  of  continued  contractions,  called  closing  tetanus,  all 
the  time  during  which  the  current  is  passing. 


Polarising 
Current 


Excitab.  diminished 


Fig.  55. — Determination  of  excitability  of  the  myopolar  segment  during  the 

passage  of  a  current  through  a  certain  length  of  the  nerve. 
In  the  lower  figure  the  polarizing  current  is  ascending;  excitability  is  di- 
minished in  the  myopolar  segment.    In  the  upper  figure  the  polarizing  current 
is  descending;  the  excitability  of  the  myopolar  segment  is  increased. 

An  "after  tetanus"  may  also  follow  the  break  of  a  strong 
ascending  current  which  has  been  passing  for  a  considerable  time. 
With  moderate  or  usual  currents,  however,  no  apparent  change 
occurs  during  the  time  the  constant  current  passes. 

Electrotonus  and  Method  of  Its  Detection  —  A  change  never- 
theless does  occur  during  this  period  between  the  make  and  break 
of  a  constant  current  which  is  capable  of  detection  by  stimulating 
different  portions  of  the  nerve  by  an  induced  current  acting  as  an 
analyzer.  During  the  time  that  the  constant  current  is  passing, 
the  analyzer  will  detect  a  region  near  the  cathode  of  the  constant 
current  where  the  excitability  is  increased,  and  hence  a  greater 
stimulus  or  impulse  may  be  produced  by  a  submaximal  stimulus 
than  would  result  in  the  absence  of  the  constant  current.  In  the 
same  manner  a  region  near  the  anode  can  be  demonstrated  in 
which  the  irritability  is  diminished.  (Fig.  55.) 

88 


THE  NERVOUS  SYSTEM 

Catelectrotonus  and  Anelectrotonus —  The  increased  irrita- 
bility at  the  cathode  is  called  catelectrotonus  and  the  diminished 
excitability  at  the  anode  is  called  anelectrotonus.  Inasmuch  a» 
the  impulse  leading  to  contraction  at  the  make  begins  at  the  cathode 
it  may  be  said  that  the  excitability  is  due  to  a  rise  of  irritability 
at  the  cathode,  dependent  upon  the  sudden  development  of  cak 
electrotonus.  In  the  same  manner  the  rise  of  excitability  at  the 
anode  accompanying  the  break  is  due  to  sudden  passing  off  oi 
anelectrotonus.  The  above  is  true  for  currents  of  moderate 
strength.  When  stronger  currents  are  used  the  indifferent  point 
separating  the  two  regions  of  anelectrotonus  and  catelectrotonus 
from  each  other  comes  to  lie  nearer  the  cathode,  so  that  more  ana 
more  of  the  nerve  is  in  a  condition  of  anelectrotonus. 

Pfliiger's  Law  —  In  the  case,  therefore,  of  very  strong  cur- 
rents the  whole  interelectrodal  portion  of  the  nerve  is  in  a  con- 
dition of  anelectrotonus  and  the  depression  of  irritability  at  the 
anode  at  the  make  is  so  great  that  the  nerve  is  non-conductive  at 
this  point.  Consequently  when  strong  currents  are  used  to  pro- 
duce stimulation,  and  the  current  is  an  ascending  one,  no  im- 
pulse can  reach  the  muscle  at  the  make  because  the  make  excitation 
starts  at  the  cathode,  which  is  furthest  from  the  muscle.  In 
ascending  currents  it  is  blocked  by  the  high  degree  of  anelec- 
trotonus at  the  anode.  In  the  same  manner  at  the  break  of  descend- 
ing currents,  when  strong  currents  are  used  the  excitation  started 
at  the  anode  by  the  passing  off  of  anelectrotonus  cannot  descend 
past  the  cathode  where  there  is  a  swing  back  from  high  catelec- 
trotonus to  a  very  low  degree  of  irritability.  These  variations  in 
the  results  of  stimulating  nerves  with  varying  degrees  of  current 
have  been  formulated  into  a  law  called  Pfluger's  law,  namely, 
that  the  result  of  stimulating  a  nerve  varies  with  the  strength  of 
the  current  and  is  as  follows : 

ASCENDING  DESCENDING 

make  break  make  break 

weak  c  0  c  0 

medium  c  c  C  c 

strong  0  C  or  T  C  or  T  0 

anelectrotonus  block     • 

(C  —  strong  contraction.  c  —  contraction.  T  —  tetanus.  0  —  no 
effect.) 

90 


THE  NERVOUS  SYSTEM 

For  these  reasons,  when  dealing  with  induced  shocks  we  are 
only  dealing  with  make  stimuli.  The  contraction  at  the  break  of 
the  primary  circuit,  which  is  evoked  by  the  make  of  the  make  and 
break  produced  in  the  secondary  circuit,  is  stronger  than  the 
contraction  produced  at  the  make  of  the  primary,  because  the  rise 
of  current  at  the  make  in  the  primary  is  a  much  slower  change 
than  the  fall  of  current  at  the  break.  In  other  words  the  intensity 
of  the  currents  induced  in  the  secondary  coils  is  proportional  to 
the  r.ate  of  change  of  the  current  in  the  primary  coil. 

Application  to  the  Human  Being  —  These  results  cannot  be 
ai  plied  to  human  nerves  with  the  same  exactness,  because  of  the 


Fig.  56. — Diagram  showing  the  internal  polarization  of  the  tissues. 
All  along  the  lines  of  the  flow  of  the  current,  going  from  one  pole  to  the 
other,  secondary  polarities  are  developed  across  the  heterogeneous  portions, 
traversed  by  electrolytic  conduction. 

impossibility  of  the  direct  application  of  the  electrodes  to  the 
human  nerves.  It  is  usual  to  apply  one  electrode  (a  stimulating 
effect  being  most  readily  obtained  when  the  stimulating  electrode 
is  the  cathode)  over  some  point  at  which  the  various  motor  nerves 
lie  nearest  to  the  surface.  The  other  electrode,  the  indifferent  one, 
is  applied  over  some  other  region  of  the  body.  Inasmuch  as  the 
current,  as  it  nears  the  cathode,  becomes  concentrated  from  a 
more  diffuse  condition  at  a  distance  from  this  electrode  called 
the  peripolar  region,  the  current  is  strongest  nearest  to  the  elec- 
trode as  it  passes  across  the  nerves.  It  exists  as  opposite  signs  on 
the  two  sides  of  the  nerve  and  is  stronger  on  the  side  nearest  the 
cathode.  (Fig.  56.)  Pure  cathode  and  anode  effects  are  not  ob- 
tainable. Applying  the  current  in  the  manner  described  to  the 
human  being  gives  the  following  phenomena  in  the  order  of  the 
strength  of  contraction  produced : 

92 


THE  NERVOUS  SYSTEM, 

C  C  C  (cathode  closing,  i.e.  make  contraction) 

A  C  C  (anode  closing  contraction) 

A  0  C  (anode  opening,  i.e.  the  break  contraction) 

COG  (cathode  opening  contraction) 

Polarization  and  Its  Explanation  in  the  Extrapolar  Region  of 
the  Nerve  —  The  length  of  a  nerve  between  the  electrodes  of  a 
current  applied  to  the  nerve  is  called  the  intrapolar  or  intraelec- 
trodal  portion.  By  sensitive  galvanometers  it  may  be  shown  that 
a  current  passes  also  through  the  extrapolar  region  of  the  nerve 
during  the  passage  of  a  constant  current.  This  current  is  in  the 
direction  of  the  constant  current  which  is  applied  to  the  nerve.  It 


Nerve. 


Anelectrotonic 
current. 


Catelectrotonic 
current. 


Fig.   56a. — Polarization    of  the   nerve    in    its   two    extrapolar   segments   and 

production  of  electrotonic  currents  in  these  two  segments. 
The  middle  region  is  traversed  by  a  constant  current  (polarizing  current;). 
The  extrapolar  regions  show  currents  of  polarization  of  the  same  direction 
as  the  preceding,  but  unequal  in  intensity.     The  anelectrotonic  current  is 
more  intense  than  the  Catelectrotonic  current. 


is  a  different  current  from  the  current  of  action.  The  same  extra- 
polar  current  may  be  excited  by  the  application  of  a  constant  curT 
rent  to  an  artificial  model  of  a  nerve,  consisting  of  a  platinum  wire 
contained  within  a  tube  and  surrounded  within  the  tube  by  normal 
saline  solution  or  any  other  electrolyte.  •  It  will  not  occur,  how- 
ever, if  the  model  is  made  of  a  zinc  wire  immersed  in  a  satu- 
rated solution  of  zinc  sulphate.  The  phenomenon  of  these 
extrapolar  currents  is  purely  physical  and  depends  upon 
polarization  produced  in  the  extrapolar  regions  of  the  nerve.  The 
.jjerve  sheath  may  be  regarded  as  composed  in  part  of  a  solution 
Electrolytes.  (Fig.  56c.) 

Upon  the  sheath  in  the  neighborhood  of  the  positive  pole  of 
the  polarizing  current  negative  ions  collect  and  in  the  same  manner 

94 


THE  NERVOUS  SYSTEM 

positive  ions  collect  in  the  neighborhood  of  the  negative  pole.  In 
fact  it  is  only  due  to  the  fact  that  these  two  sets  of  ions  are  at- 
tracted to  the  poles  and  give  up  their  charges  to  them  that  any 
current  from  the  cell  passes  through  the  nerve  at  all.  These 
attracted  ions  at  the  points  of  application  of  the  electrodes  create 
in  this  region  a  difference  of  potential  which  extends  to  the  extra- 
polar  region. 

The  Current  of  Positive  Polarization  and  Its  Negative  Varia- 
tion —  In  consequence  of  this  fact  a  current  will  pass  in  the  extra- 
polar  region.  This  current  is  called  the  current  of  polarization. 
(Fig.  56a.)  If  now  the  electrodes  of  the  constant  current  are  taken 


Fig.  56b. — Post-electrotonic  intrapolar  currents  produced  after  the  cessation  of 

the  polarizing  current. 

I,  polarizing  current;  II,  ordinary  post-current  of  contrary  direction  or  the 
current  of  negative  polarization;  III,  post-current  whose  direction  is  the 
same  as  that  of  the  polarizing  current  or  the  current  of  positive  polarization. 


away  from  the  nerve  and  in  their  place  the  poles  of  a  galvanometer 
are  applied,  the  galvanometer  will  show  a  temporary  current  in  the 
opposite  direction  to  that  in  which  the  constant  current  was  pre- 
viously flowing,  though  immediately  preceding  this  current  in  the 
opposite  direction  there  will  be  momentary  current  in  the  same 
direction.  This  momentary  current  in  the  same  direction  is 
known  as  the  current  of  positive  polarization,  and  that  in  the 
opposite  direction  as  the  current  of  the  negative  variation  of  the 
polarizing  current.  The  current  of  positive  polarization  is  really 
the  current  of  action  excited  by  the  break  produced  when  the 
electrodes  are  lifted  off.  The  negative  variation  of  the  polarizing 
current  is  due  to  the  difference  of  potential  created  at  the  places 
where  the  poles  of  the  constant  or  polarizing  current  had  collected 
ions  of  unlike  sign  to  that  of  the  poles  of  the  polarizing  current 
in  the  region  of  the  poles.  Inasmuch  as  the  ions  thus  collected 
at  these  spots  are  of  unlike  sign  to  that  of  the  poles,  the  current 
will  flow  in  the  opposite  direction  to  that  of  the  polarizing  cur- 

96 


THE  NERVOUS  SYSTEM 

rent.     (Figs.  56b  and  56c.)     These  currents  of  polarization  are 
called  electrotonic  currents. 

The  electrotonic  current  excited  in  one  of  the  two  main  branches 


-i-  - 


Fig.  56c. — Diagram  illustrating  the  position  of  the  ions  which  are  responsible 
for  the  direction  of  the  current  of  negative  polarization 

indicated  by  the  galvanometer,   G. 

When  the  current  ceases  to  flow  through  the  battery  circuit,  B,  the  col- 
lection of  a  larger  number  of  positive  ions  in  the  neighborhood  of  the  pole,  C, 
causes  the  current  to  flow  in  the  reverse  direction  through  the  galvanometer. 
The  plus  and  minus  signs,  D  and  E,  indicate  the  direction  of  extra  polar 
electrotonic  currents. 

of  a  sciatic  nerve  is  strong  enough  to  excite  an  effective  impulse 
in  the  other  branch.  This  effect  is  not  due  to  spreading  of  the 
current,  as  it  will  not  occur  if  the  stimulated  branch  is  crushed. 


THE  PERIPHERAL  NERVES 


CONDITIONS  AFFECTING  ELECTRICAL  STIMULATION  OF  THE  NERVES 


The  Speed  of  the  Make  or  Break  —  An  apparatus  has  been 
devised  which  depends  upon  the  rapid  or  slow  opening  of  a  shutter 
controlling  the  size  of  an  opening  between  two  concentrated  solu- 

98 


THE  NERVOUS  SYSTEM 

tions  of  zinc  sulphate  in  which  is  immersed  electrodes  of  zinc  and 
which  is  capable  of  more  rapidly  or  slowly  increasing  the  current. 
-(Fig.  57.) 

i 


Fig.  57. — A,  Diagram  of  Rheonome.  By  means  of  the  shutter,  1,  the  speed 
of  the  make  and  break  of  the  current  between  the  electrolytes  in  the  two 
boxes,  2  and  3,  communicating  only  through  the  slit,  4,  may  be  varied. 

B,  Illustrating  the  galvanometer  records  of  the  change  of  the  current  obtained 
by  the  differences  of  the  speed  of  the  make  accomplished  by  the  Rheonome. 

The  excitatory  effect  varies  in  proportion  to  the: 

(1)  Intensity  of  the  current. 

(2)  And  with  certain  limitations  upon  the  rate  of  change 
of  the  current.    Rapid  alterations  in  the  current,  as  in  very  rapid 
alterations  of  the  induced  current,  are  ineffective.    It  is  for  this 
reason  that  the  high  voltage  of  the  Tesla  current  may  be  used  for 
therapeutic  effects  upon  the  human  being. 

100 


THE  NERVOUS  SYSTEM 

Waller's  Characteristic  —  The  optimum  rate  of  change  is 
known  as  Waller's  characteristic.  The  rate  of  change  may  be 
recorded  graphically  and  is  called  the  current  gratient. 

The  Duration  of  a  Current  which  is  used  to  produce  excitation 
also  bears  important  relations  to  the  strength  of  excitation.  This 
relation  may  be  investigated  by  first  ascertaining  the  minimal 
strength  of  current  necessary  to  produce  excitation  and  then  deter- 
mining how  much  the  current  must  be  increased  as  the  time  be- 
tween the  make  and  break  is  shortened  to  produce  the  same  con- 
traction. 

Keith  Lucas'  Characteristic  —  Keith  Lucas  has  used  as  a  second 
characteristic  that  duration  of  the  stimulus  which  will  just  pro- 
duce an  excitation  when  the  strength  of  a  minimal  effective  stimulus 
is  just  doubled.  Each  tissue,  muscle,  nerve  and  nerve  ending  as 
well  as  these  various  tissues  in  different  animals  have  all  their 
definite  various  characteristics. 

The  Effect  of  Temperature  upon  Excitability  —  Within  certain 
limits  the  excitability  of  a  nerve  may  be  increased  by  cooling. 
Thus  a  frog's  nerve  cooled  to  2°  C.  for  a  day  will  be  so  excitable 
that  simple  section  of  the  nerve  may  be  sufficient  to  throw  it  into 
a  tetanus.  This,  however,  is  only  true  for  mechanical  stimuli.  In 
the  case  of  electrical  stimuli  warming  of  the  nerve  increases  irri- 
tability and  cooling  diminishes  it  for  all  galvanic  or  induction 
shocks  of  less  duration  than  .005  of  a  second.  In  the  case  of 
skeletal  muscles  excitability  is  increased  by  cooling  for  all  forms 
of  stimuli. 

The  reason  why  the  time  of  .005  of  a  second  is  a  factor  in  the 
effect  of  temperature  on  irritability  is  because  the  temperature 
produces  two  effects  which  do  not  vary  at  the  same  rate  with 
changes  of  temperature.  Thus  cooling  of  a  nerve  both  delays  the 
subsidence  of  the  excitatory  process  and  renders  more  difficult  the 
initiation  of  the  excitatory  change,  but  the  delay  in  the  subsidence 
of  the  excitatory  process  reduces  the  amount  of  current  needed 
for  excitation  in  an  increasing  ratio  the  longer  the  duration  of  the 
current,  while  increasing  the  difficulty  of  the  initiation  of  the  exci- 
tatory process  such  as  is  produced  by  (1)  cooling,  (2)  prolonging 
the  current,  (3)  delaying  its  subsidence,  increases  also  the  current 
required  for  excitation  in  the  same  ratio,  or  in  an  exponential 
ratio  as  the  current  is  lengthened. 

102 


THE  NERVOUS  SYSTEM 

Effect  of  Injury  on  Irritability  —  Immediately  after  injury  a 
nerve  is  more  irritable  near  the  site  of  injury.  After  a  time  irri- 
tability diminishes  progressively  in  a  downward  direction,  so  that 
the  portion  nearest  the  muscle  is  longest  irritable. 

Direction  of  Conductivity  Across  the  Motor  End  Plate  —  The 
motor  end  plate,  or  the  interval  between  the  nerve  and  the  muscle, 
constitutes  a  true  synapse.  Conduction  across  it  is  only  possible 
in  one  direction.  This  is  proved  by  the  purely  local  contraction 
which  is  excited  by  snipping  a  nerve-free  portion  of  a  split  muscle 
and  contrasting  the  local  contraction  so  excited  with  the  contrac- 
tions in  both  split  portions  when  the  ends  of  the  nerves  in  one  half 
are  snipped. 

The  Specific  Delay  at  the  End  Plate  —  A  certain  definite  period 
of  delay  exists  in  the  transmission  of  an  impulse  across  a  motor 
end  plate.  It  has  been  found  to  be  -.0013  of  a  second.  The  motor 
end  plate  also  shows  fatigue  more  quickly  than  either  the  nerve 
or  muscle,  a  fact  also  indicated  by  the  specific  action  of  curare  and 
nicotine  upon  the  motor  end  plate. 

Action  of  Nicotine  —  Nicotine,  after  a  primary  stimulant 
action,  has  very  much  the  same  blocking  effect  upon  the  motor 
end  plate  that  curare  possesses,  though  it  is  much  less  powerful 
than  curare.  Four  mg.  of  nicotine  injected  into  the  veins  of  a 
anesthetized  fowl,  will  cause  a  tonic  contraction  of  all  the  muscles, 
a  phenomenon  which  may  be  immediately  set  aside  by  curare  and 
one  which  can  occur  when  enough  nicotine  has  been  given  to 
paralyze  all  the  motor  nerves,  or  after  all  the  motor  nerves  have 
degenerated  in  consequence  of  their  having  been  previously  sec- 
tioned. Thus  nicotine,  like  curare,  produces  its  effect  by  acting 
upon  the  substance  of  the  motor  end  plates. 

Specific  Optimal  Excitation  Time  of  the  Motor  End  Plate  — 
A  fourth  fact  demonstrating  the  different  essence  of  the  motor 
end  plate  is  its  different  optimal  excitation  time.  This  represents 
the  relation  of  the  strength  of  the  current  to  the  duration  of  the 
current  necessary  to  produce  contraction.  The  muscle  and  the 
nerve  and  motor  end  plate  all  possess  different  optimal  excita- 
tion times.  The  nerve-free  end  of  the  sartorious  possesses  an 
excitation  time  of  .017  seconds,  and  this  may  be  taken  as  the  excita- 
tion time  of  the  muscle.  % 

The  excitation  time  of  the  sciatic  nerve  trunk  is  about  .003 

104 


THE  NERVOUS  SYSTEM 

second,  and  possesses  a  steeper  gradient.  That  then  Is  the  excita- 
tion time  of  nerve  fibers.  In  the  middle  of  the  sartorious  muscle 
of  the  frog,  in  the  region  of  the  motor  end  plate,  the  excitation  time 
is  .00005. 

The  Neuromuscular  Juncture  of  the  Sympathetic  Nerves  and 
the  Action  of  Adrenalin  upon  It  —  The  sympathetic  nerve  fibers 
end  seemingly  in  direct  contact  with  the  muscular  fibers,  without 
the  intermediation  of  any  motor  end  organ.  Nevertheless  there 
is  evidence  that  here  also  there  is  present  a  third  substance  dif- 
ferent from  the  nerve  and  muscle  which  bridges  the  gap  between 
the  two  though  it  has  not  organization,  at  least  of  a  motor  end  plate. 
Adrenalin  possesses  a  specific  stimulant  action  on  the  whole  of  that 
portion  of  the  sympathetic  nervous  system  which  causes  augmenta- 
tion of  function.  Hence  it  contracts  all  blood  vessels  supplied  by 
these  nerves.  Smooth  muscle  not  innervated  by  the  sympathetic 
nervous  system  as  that  of  the  blood  vessels  of  the  brain  and  lungs  is 
unaffected  by  the  injection  of  adrenalin.  Therefore  the  action  of 
adrenalin  cannot  be  upon  the  muscular  fiber  itself.  Adrenalin  is 
just  as  effective,  however,  after  complete  degeneration  of  the  post- 
ganglionic  fibers  of  the  sympathetic  nerves,  so  that  its  action  cannot 
be  upon  these  post-ganglionic  fibers  themselves,  but  must  be  upon 
some  third  substance  intervening  between  this  fiber  and  the  muscle. 
The  same  substance  may  be  intermediary  in  all  synapses  and  closely 
allied  to  the  substance  of  the  motor  end  plate.  Its  similar  nature 
to  the  material  of  the  motor  end  plate  is  suggested  by  the  fact  that 
injections  of  adrenalin  act  variously  on  skeletal  muscle,  forming 
a  marked  contrast  to  barium,  which  stimulates  every  skeletal 
muscle  fiber  in  the  body,  acting  upon  them  directly. 

The  Facts  Indicating  the  Nature  of  an  Excitatory  Process  — 
By  the  following  facts  the  nature  of  an  excitatory  process  in  nerves 
is  indicated: 

(1)  The  dependence  of  irritability  of  a  nerve  upon  a  supply 
of  oxygen  demonstrating  that  there  is  an  expenditure  of  energy 
even  though  in  medullated  fibers  evidence  of  fatigue  is  absent. 
It  is  impossible  to  estimate  any  dissipation  of  energy  in  the  form 
of  heat.    Non-medullated  fibers  can  be  fatigued. 

(2)  The  failure  of  the  decrement  in  the  excitatory  process  as 
it  becomes  transmitted. 

(3)  The  excitatory  state  is  attended  with  an  electrical  change 

106 


THE  NERVOUS  SYSTEM 

of  such  a  nature  that  an  excited  spot  is  negative  to  all  other 
spots.  The  electrical  change  rises  to  a  maximum  rapidly  and  dies 
away  slowly.  The  amount  of  rise  and  fall  is  dependent  upon  the 
tissue  under  investigation. 

(4)  The  excitatory  change  is  aroused  only  at  the  poles  of  a 
current  passing  through  the  tissue,  i.e.,  at  the  places  where  the 
collection  of  ions  is  greatest. 

(5)  Excitation  occurs  only  at  the  cathode  at  the  make  and, 
only,  if  the  current  attains  sufficient  strength  within  a  certain 
length  of  time. 

(6)  All  living  tissues  are  made  up  of  colloids  divided  into 
compartments  by  membranes  of  various  permeabilities  and  perme- 
ated with  salts  and  various  electrolytes  in  solution.    Very  many 
possibilities,  therefore,  exist  for  the  formation  of  successive  com- 
partments characterized  by  large  differences  in  potential.     We 
may  conceive  of  a  successive  transmission  of  such  electrical  states, 
and  of  such  a  movement  of  the  ions,  along  a  successive  series  of  com- 
partments in  a  nerve  fiber  as  will  account  for  large  differences  in 
potential.     The  process  may  be  roughly  likened  to  the  explosion 
of  a  successive  chain  of  equal  masses  of  gunpowder.    The  number 
of  variables  affecting  the  movement  of  the  ions  are  numerous  and 
will  permit  of  many  possibilities. 


108 


II 
THE  SPINAL  CORD 

MORPHOLOGY 

Its  Structure  from  a  Development  Standpoint  —  In  certain 
orders  of  invertebrates  the  whole  central  nervous  system  is  com- 
posed solely  of  ganglia  united  by  nerve  strands.  These  simpler 
nervous  systems  are,  therefore,  segmental  in  character.  Even  in 
the  embryo  of  mammals  the  first  traces  of  the  nervous  system  are 
segmental.  In  the  fully  developed  nervous  system  of  mammals  all 
trace  of  the  segmental  character  of  the  nervous  system  is  lost 
except  as  it  is  represented  by  the  regular  manner  in  which  the 
spinal  nerves  leave  the  spinal  cord. 

Gross  Anatomy  of  the  Spinal  Cord  —  The  spinal  cord  is  ap- 
proximately eighteen  inches  long.  (Fig.  58.)  From  its  lateral 
surfaces  are  given  off  thirty-one  pairs  of  nerves.  Each  pair  is  com- 
posed of  an  anterior  and  posterior  root  which  emerges  from  the 
spinal  canal  through  the  intervertebral  foramen.  On  each  posterior 
root,  before  it  joins  the  anterior  root  in  its  passage  through  the 
intervertebral  foramen,  is  a  large  ganglion  through  which  the  pos- 
terior root  passes.  The  anterior  roots  arise  by  a  series  of  fasciculi 
spread  out  over  a  rather  considerable  length  of  the  particular  level 
of  the  cord  from  which  the  root  arises.  The  posterior  root  arises 
as  a  single  well-marked  bundle. 

On  section  of  the  cord  it  is  seen  to  be  composed  of  a  peripheral 
white  substance  and  a  central  gray  substance.  The  central  gray 
substance  is  shaped  somewhat  like  an  H,  and  possesses  in  other 
words  two  anterior  horns  and  two  posterior  horns  connected  with 
a  transverse  bar  of  gray  matter.  (Fig.  59.)  In  the  center  of  this 
transverse  bar  is  a  central  canal.  The  white  matter  owes  its  color 
to  the  fact  that  it  is  composed  of  medullated  nerve  fibers.  The  gray 
matter  is  darker  because  within  it  are  contained  the  nerve  cells  of 
the  spinal  cord.  Each  lateral  half  of  the  spinal  cord  is  separated 

110 


THE  NERVOUS  SYSTEM 


Middle ' 
peduncle. 
Inferior 
peduncle. 


VII. 


VIII. 


Superior     peduncle    of 

the  cerebellum. 
Sulcus       longitudinalis 

medius. 

Glosso-pharyngeal. 
Vagus. 

Spinal  accessory. 


Ligamentum  denticula- 
tum. 

Posterior     longitudinal 
fissure. 


An  anterior  root. 
A  posterior  root. 


XII.— 


L.  I 


S.  I 


Ganglion  on 
a  posteri- 
or root. 


Fig.  58. — Dorsal  aspect  of  the  medulla  oblongata  and  spinal  cord  with  the  dura 
mater  partially  removed.    (Morris.) 


112 


THE  NERVOUS  SYSTEM 


114 


THE  NERVOUS  SYSTEM 


from  the  other  by  a  deep  anterior  and  posterior  fissure  which 
reaches  nearly  to  the  central  transverse  bar  of  gray  matter.    At  the 
bottom  of  these  fissures  are  strands  of  transverse  white  fibers. 
These  are  the  anterior  and  posterior  commissures.    The  amount 

of  central  gray  matter  at  any 
level  of  the  spinal  cord  is  in 
proportion  to  the  number  of 
nerve  fibers  coming  off  at  that 
particular  level.  The  amount 
of  white  matter  diminishes 
progressively  from  above 
downwards.  The  size  of  the 
transverse  section  of  the  cord 
is  greatest  in  the  cervical  re- 
gion, that  region  supplying 
the  upper  limbs  and  contain- 
ing all  the  fibers  in  the  white 
matter  to  and  from  the  dorsal 
and  lumbar  regions  as  well. 

The  cord  is  smallest  in  the 
dorsal  region  and  again  en- 
larges in  the  lumbar  region 
though  smaller  here  than  in 
the  cervical  region.  The  en- 
largement in  the  lumbar  re- 
gion is  dependent  entirely 
upon  the  large  amount  of 
gray  matter,  the  axons  of 
which  supply  the  lower  limbs. 
(Fig.  60.) 

The  Group  of  Nerve  Cells 
—  The  nerve  cells  of  the  gray 
matter  are  collected  into  cer- 
tain groups.     (Fig.  61.)    Of  the  anterior  horn  there  are : 

1.  A  median  group.    Many  of  the  processes  of  this  group  cross 
the  middle  line. 

2.  An  external  group  of  large  multipolar  cells  whose  axons 
enter  directly  the  anterior  nerve  roots. 

3.  At  the  base  of  the  anterior  horn,  in  a  region  which  may  be 

116 


Fig.  60. — Sections  of  spinal  cord  in  lower 
cervical,  mid-thoracic,  and  mid- 
lumbar  regions.     (Quain.) 
On  the  right  side  of  each  section  con- 
ducting tracts  are  indicated.    P.-M!  (in 
the  lumbar  region) ,  septo-marginal  tract. 


THE  NERVOUS  SYSTEM 


termed  the  lateral  horn,  is  a  group  of  small  cells  whose  fibers 
also  enter  the  anterior  nerve  roots,  forming  the  smaller  nerve  fibers 
of  these  roots  and  belonging  to  the  sympathetic  nervous  system. 

4.  In  the  posterior  horn  the  nerve  cells  are  more  scattered  but  a 
very  well-marked  collection  of  cells  exists  at  the  root  of  the  pos- 
terior horn  near  its  in- 
ternal side.  It  is  known 
as  Clark's  column  of 
cells. 

The  cells  of  the  gray 
matter  of  the  spinal  cord 
may  be  classified  from 
their  functional  stand- 
point as : 

1.  Motor  cells,  chief- 
ly the  a  n  t  e  r  o-lateral 
cells  and  the  cells  of  the 
anterior  horn. 

2.  Cells  of  the  col- 
umns.    The   cells   of 
Clark's  column,  because 
their  axons  form  a  defi- 
nite column  in  the  lat- 
eral regions  of  the  white 
matter    of    the    spinal 
cord. 

3.  Commissural  cells 
whose  axons  cross  to  the 
opposite  side  of  the  cord. 

4.  Cells    of    Golgi, 
represented  by  a  large 

number  of  the  cells  of  the  posterior  horn  which  are  multipolar 
and  whose  axons  do  not  travel  far  from  the  cell  but  rapidly  break 
up  into  dendrites.  They  are,  therefore,  chiefly  associative  in 
function. 

Significance  of  the  Connections  of  the  System  of  Neurons  — 
For  a  proper  understanding  of  the  functions  of  the  spinal  cord  a 
knowledge  of  the  connections  of  the  tracts  which  form  systems  of 
neurons  is  absolutely  essential. 

118 


Fig.  61. — Diagram  showing  on  the  right  side 
the  "ascending"  and  on  the  left  side  the 

"descending"  tracts  in  the  spinal  cord. 
1,  Crossed  pyramidal;  2,  direct  pyramidal; 
3,  antero-lateral  descending;  3a,  bundle  of 
Helweg;  4,  prepyramidal ;  5,  comma;  6,  pos- 
tero-mesial;  7,  postero-lateral ;  8,  marginal; 
9,  dorsal  cerebellar;  10,  antero-lateral  ascend- 
ing or  ventral  cerebellar;  s-m,  septo-marginal ; 
s.p.L,  superficial  postero-lateral  fibres  (dorsal 
root-zone  of  Flechsig) ;  o  to  a5,  groups  of  cells 
in  the  anterior  horn;  i,  intermedio-lateral 
group  or  cell-column  in  the  lateral  part  of  the 
grey  matter;  p,  cells  of  posterior  horn;  d 
dorsal  nucleus  or  cell-column  of  Clark.  The 
dots  represent  "endogenous"  fibres  (arising  in 
grey  matter  of  cord)  having  for  the  most  part 
a  short  course. 


THE  NERVOUS  SYSTEM 


Methods  for  Tracing  the  System  of  Neurons  — 

a.    By  intravital  staining. 

&.  By  the  impregnation  method  of  Golgi.  This  method  accom- 
plishes the  impregnation  of  the  neurons  by  a 
silver  salt  which  blackens  on  exposure  to  light. 
The  feature  which  makes  this  method  of  value 
is  the  fact  that  the  impregnation  is  not  general. 
In  virtue  of  this  fact  sections  of  considerable 
thickness,  containing  a  long  distance  of  any 
nerve  fiber,  may  be  studied. 

c.  Myelination   Method  —  As   the   nervous 
system  develops  the  nerve  fibers  become  inclosed 
in  their  myelin  sheaths.    The  various  systems  of 
neurons  do  not  acquire  their  sheaths  at  the  same 
period.    The  tracts,  which  develop  phylogeneti- 
cally  later,  that  is  the  youngest  tracts,  acquire 
their  myelin  sheaths  later  and  may  hence  be 
recognized  by  stains  which  bring  out  the  myelin 
sheath,  such  stains  when  applied  at  an  early 
period  of  their  development  failing  to  render 
them  conspicuous  as  compared  to  the  other  sur- 
rounding tracts. 

d.  The  Walletian  Method  —  A  nerve  fiber 
which  is  divided   degenerates   away   from  the 
nerve  cell  of  which  the  fiber  may  be  an  axon. 
Whole  tracts  of  nerve  fibers  may,  therefore,  de- 
generate when  they  come  from  the  same  area  of 
gray  matter.     (Fig.  62.)     As  a  result  of  this 
degeneration  fatty  products  are  formed  in  the 
myelin  sheath  of  the  degenerated  nerves  which 
take  an  intense  black  stain  with  osmic  acid.    At 
a  period  of  three  weeks  after  the  tract  has  been 
divided  it  will  appear  black;  at  the  end  of  six 
months  these  products  will  be  removed  and  the 
degenerated  tract  will  take  no  stain  at  all.    At 
this  stage  it  will  also  contrast  strongly  with  the 

undegenerated  tracts  which  stain  normally.     (Fig.  63.) 

e.    Method  of  Retrograde  Degeneration  —  If  an  axon  is  divided 
at  a  point  distal  to  its  cell,  a  certain  degree  of  degeneration  will 

120 


Fig.  62.— Diagram 
showing  the  de- 
scending degen- 
eration of  the 
pyramidal  tract 
following  a  le- 
sion in  the  left 
c  e  r  e  b  r  o  hemi- 
sphere involving 
the  Rolandic 
area. 


THE  NERVOUS  SYSTEM 

appear  in  the  cell  for  a  period  of  a  few  weeks,  after  which  such  a 
cell  will  again  regain  its  normal  staining  power. 


vn. 


vm. 


Fig.  63. — Diagram  of  sections  of  the  spinal  cord  of  the  monkey,  showing  the 
position  of  degenerated  tracts  of  nerve-fibres  after  specific  lesions  of  the 
cord  itself,  of  the  afferent  nerve-roots,  and  of  the  motor  region  of  the  cere- 
bral cortex.  The  degenerations  are  shown  by  the  method  of  Marchi.  The 
left  side  of  the  cord  is  in  all  cases  on  the  reader's  left  hand.  (Quain.) 

I.  Degenerations  resulting  from  extirpation  of  the  motor  area  of  the  cor- 
tex of  the  left  cerebral  hemisphere. 

II.  Degenerations  produced  by  section  of  the  dorsal  longitudinal  bundles 
in  the  upper  part  of  the  medulla  oblongata. 

III.  and  IV.    Results  of  section  of  dorsal  roots  of  the  first,  second,  and 
third  lumbar  nerves  on  the  right  side.    III.  is  from  the  segment  of  cord  be- 
tween the  last  thoracic  and  first  lumbar  roots;  IV.  from  the  same  cord  in  the 
cervical  region. 

V.  to  VIII.  Degenerations  resulting  from  (right)  semi-section  of  the  cord 
in  the  upper  thoracic  region.  V.  is  taken  a  short  distance  above  the  level  of 
section;  VI.  higher  up  the  cord  (cervical  region);  VII.  a  little  below  the 
level  of  section;  VIII.,  lumbar  region. 

122 


THE  NERVOUS  SYSTEM 

The  same  degenerative  changes  may,  however,  appear  in  a 
motor  cell  from  a  section  of  posterior  nerve  root.  Retrograde  de- 
generation is  probably,  therefore,  a  degeneration  of  disuse  and  must 
be  used  in  the  tracing  out  of  nerve  tracts  with  much  caution. 

PHYSIOLOGY 

THE  EFFERENT  AND  AFFERENT  PATHS  TO  AND  FROM  THE  CORD 

The  Functions  of  the  Anterior  Nerve  Roots  and  Method  of 
Investigating  Them  —  If  the  anterior  nerve  root  is  divided  there 
will  result  a  paralysis  of  certain  muscles.  If  the  central  end  of 
the  root  is  stimulated  no  effects  will  be  produced.  If,  on  the  other 
hand,  the  peripheral  end  is  stimulated  certain  muscles  will  con- 
tract. 

If  the  anterior  root  is  one  of  the  thoracic  roots,  certain  visceral 
effects  will  be  produced  by  peripheral  stimulation.  Dilatation  of 
the  pupils,  or  constriction  of  certain  blood  vessels,  or  augmentation 
of  the  heart  beat,  are  among  these  visceral  effects. 

The  Functions  of  the  Posterior  Roots  —  Division  of  one  pos- 
terior nerve  root  will  usually  produce  no  noticeable  effect.  Stimu- 
lation of  its  peripheral  end  is  also  without  effect. 

Stimulation,  however,  of  its  central  end  in  the  conscious  animal 
will  produce  signs  of  pain.  If  the  spinal  cord  has  been  divided 
beneath  the  brain,  central  stimulation  will  produce  reflex  move- 
ments. If  two  or  three  posterior  nerve  roots  have  been  divided 
there  will  be  anesthesia  over  limited  areas  of  the  surface. 

The  anterior  roots  must,  therefore,  be  regarded  as  entirely 
efferent  in  their  function,  i.e.,  as  the  pathway  out  from  the  spinal 
cord,  the  posterior  roots  as  entirely  afferent  and  the  pathway  into 
the  spinal  cord. 

The  Peripheral  Distribution  of  the  Anterior  Roots,  and  the 
Function  of  the  Plexuses  —  Each  muscle  of  the  limbs  receives 
nerve  fibers  from  more  than  one  segment  of  the  spinal  cord.  Hence 
stimulation  of  one  anterior  nerve  root  in  a  peripheral  direction 
does  not  produce  contraction  of  any  one  muscle  or  one  physiological 
group  of  muscles.  The  anterior  nerve  roots  passing  from  that 
region  of  the  spinal  cord  which  supplies  the  limbs  unite  after  leav- 
ing the  spinal  cord  to  form  plexuses.  From  these  plexuses  the 

124 


THE  NERVOUS  SYSTEM 

single  nerves  come  off  which  supply  groups  of  muscles  which  are 
physiologically  related. 

Stimulation  of  one  of  these  nerves,  therefore,  causes  a  con- 
traction of  one  muscle  or  one  group  of  muscles  which  are  physio- 
logically related. 

The  Function  of  the  Fine  Fibers  —  A  section  of  a  thoracic 
anterior  nerve  root  shows  it  to  be  composed  of  large  and  small 
fibers.  The  large  fibers  are  axons  of  the  large  cells  in  the  lateral 
region  of  the  anterior  horn  and  run  to  the  muscles. 

The  fine  fibers  are  axons  of  the  cells  in  the  lateral  horns  and 
pass  as  the  white  rami  communicates  to  the  sympathetic  system. 
These  fibers  transmit  impulses  which  cause  dilatation  of  the  pupils, 
augmentation  of  the  heart  beat,  contraction  of  the  blood  vessels, 
inhibition  of  movements  of  the  intestines  and  erection  of  hairs. 

1.  The  Immediate  Fate  of  the  Fibers  of  the  Posterior  Nerve 
Roots  —  The  fibers  of  the  posterior  roots  pass  directly  into  the 
spinal  cord,  divide  into  an  ascending  bundle  and  descending 
bundle. 

(a)  Lissauer's  Column  —  The  descending  bundle  forms  a  tract 
near  the  tip  of  the  posterior  horn.     It  is  known  as  Lissauer's 
column.    The  fibers  of  this  tract  are  short  and  terminate  in  cells 
in  the  substantia  gelatinosa  at  the  tip  of  the  posterior  horn. 

(b)  The  Columns  of  Goll  and  Burdach  —  The  ascending  fibers 
are  long.    A  large  number  of  them  pass  upwards  in  the  posterior 
white  columns  of  the  cord,  i.e.,  in  the  region  between  the  posterior 
horns  and  the  posterior  median  fissure.    In  these  columns  the  fibers 
ascend  through  the  whole  length  of  the  cord,  and  as  they  ascend 
they  occupy  a  more  median  position,  making  room  in  this  manner 
laterally  for  other  fibers  entering  at  higher  levels.    (Fig.  64.)    The 
external  half  of  this  posterior  column  is  called  the  column  of  Bur- 
dach, the  internal  bundle  is  called  the  column  Goll.    Both  are  well 
marked  in  the  cervical  region.    Other  ascending  fibers  terminate  at 
different  levels  of  the  spinal  cord  around  cell  in  the  posterior  horn. 
All  of  the  long  fibers  give  off  collateral  fibers  to  cells  at  different 
levels  of  the  spinal  cord.    The  fibers  forming  these  long  tracts  are 
the  most  internal  of  the  fibers  of  the  posterior  root. 

(c)  Fibers  ending  in  the  cell  tracts  of  the  gray  matter.    These 
fibers  occupy  a  position  in  the  posterior  root  between  the  fibers 
of  the  long  posterior  columns  and  those  of  Lissauer's  column. 

126 


^  9-il  i 


i  linn 


THE  NERVOUS  SYSTEM 

Five  groups  are  traceable.     (Figs.  64,  66,  67  and  68.) 

1.     Fibers  to  the  cells  of  the  posterior  horn  of  the  same  side. 


Fig.  65. — Cross-sections  of  the  spinal  cord  of  the  dog  revealing  the  position 
of  the  nerve-tracts  descending  to  the  hind-limb  region  from  origin  in  the 
foremost  three  thoracic  segments,  by  the  method  of  "successive  degenera- 
tion." 

The  eighth  cervical  segment  had  been  exsected  and  568  days  later  a  crosscut 
was  made  at  the  hindmost  level  of  the  third  thoracic  segment.  The  trans- 
verse extent  of  this  lesion,  as  determined  by  microscopic  sections  afterwards, 
is  shown  in  diagram  1  of  the  figure.  The  greater  part  of  the  right  lateral 
column  is  seen  to  have  been  spared  from  injury.  Three  weeks  subsequent  to 
this  second  lesion  the  animal  was  sacrificed.  Preparations  made  with  the 
Marchi  method  for  revealing  degenerate  nerve  fibres  showed  the  degeneration 
indicated  by  diagrams  2,  3,  4  and  5  in  the  figure.  After  the  second  injury  to 
the  cord  the  scratch-reflex  remained  elicitable  from  the  right  shoulder,  but 
was  lost  from  the  left  shoulder  in  its  anterior  scapular  region.  The  degenera- 
tion of  these  proprio-spinal  fibres  descending  from  the  shoulder  segments  went, 
therefore,  hand  in  hand  with  disappearance  of  the  scratch-reflex  from  a  region 
of  skin  of  the  shoulder  whence  it  was  elicitable  previously.  (Sherrington.) 

130 


Fig.  66. — Diagram  representing  the  manner  of  origin  or  termination  of  the 
roots  of  the  spinal  nerves  in  the  gray  matter  substance  of  the  spinal  cord. 
The  distribution  of  the  cells  of  the  gray  matter  and  the  tracts  of  nerve 
fibres  in  the  white  substance  of  the  spinal  cords: 

1.    THE  ROOTS 

r,  the  cells ;  ra,  the  fibres  of  the  .anterior  nerve  roots ;  rpe,  external  trunk ; 
rpi,  internal  trunk  of  a  posterior  nerve-root  with,  n,  collaterals  going  to  a 
posterior  external  group  of  cells  of  the  anterior  horn  and,  r,  to  a  group  an- 
terior to  the  latter;  ss,  to  posterior  horn  cells,  and,  t,  to  the  column  of  Clark. 

2.      THE   GRAY   MATTER 

a,  cells  of  the  lateral  column;  b,  cells  of  the  posterior  column;  c,  cells,  the 
axons  of  which  cross  the  white  commissure;  cp.  posterior  commissure;  d  (in 
the  substance  of  Rolando),  cells  of  Golgi  with  short  axons;  k,  cells,  the  axons 
of  which  cross  the  posterior  commissure  and  go  to  the  posterior  horn  of  the 
opposite  side;  r,  cells,  the  axons  of  which  turn  back  into  the  posterior  root 
zone. 

3.      COLUMNS  OF  THE  WHITE  MATTER 

al,  antero-lateral  column  or  the  column  of  Gowers;  am,  antero-marginal 
column  or  the  column  of  Loewenthal;  bpa,  bpm,  brs,  median  portion,  antero 
and  posterior  columns  of  Burdach;  cl,  direct  cerebellar  tract;  jb,  continuation 
of  the  posterior  longitudinal  bundle;  jbl,  fibres  of  the  lateral  column;  fga, 
fibres  descending  possibly  from  the  anterior  corpora  quadrigemina,  and  pass- 
ing across  the  base  of  the  commissure;  fl,  fibres  of  the  olivo-spinal  tract;  fla, 
fibres  of  the  anterior  columns;  jp,  fibres  of  the  antero-internal  column;  ft, 
fibres  of  the  prepyramidal  tract  of  Monokaw;  gi,  portion  of  the  column  of 
Goll  contiguous  to  the  dorsal  septums;  gl,  intermediate  zone  of  the  posterior 
column;  i,  fibres  of  the  median  or  deep  lateral  column;  I,  fibres  coming  from 
the  lateral  column  and  terminating  in  the  anterior  horn;  m,  fibres  from  the 
anterior  root  zone;  n,  fibres  from  the  deep  column  terminating  in  the  pos- 
terior horn;  p  lateral  pyramidal  tract  containing  two  kinds  of  fibres;  p,  an- 
tero-pyramidal  tract;  cp,  ventral  zone  of  the  posterior  tract;  z,  external  or 
marginal  root  zone. 

132 


THE  NERVOUS  SYSTEM 

2.  Fibers  to  the  cells  of  the  posterior  horn  of  the  opposite 
side. 

3.  Fibers  to  the  median  group  of  cells  of  the  anterior  horn. 

4.  Fibers  to  the  cells  of  Clark's  column. 

5.  Fibers  to  the  motor  cells  of  the  anterior  horn. 
Significance  of  the  Distribution  of  the  Fibers  of  the  Posterior 

Nerve  Roots  —  Means,  therefore,  exist  by  which  an  incoming  im- 
pulse may  pass  upward  for  the  whole  length  of  the  spinal  or  down- 
wards or  upwards  for  several  or  many  segments  of  the  spinal  cord 
or  to  a  number  of  different  groups  of  nerve  cells  within  the  gray 
matter. 

2.  Inter segmental  Filters  and  Their  Positions  (Fig.  65)  —  The 
fibers  passing  between  different  segments  of  the  spinal  cord  are  of 
much  importance.  Within  the  white  matter  of  the  spinal  cord  they 
occupy  the  following  regions : 

a.  In  the  lateral  columns  immediately  outside  the  central  gray 
matter  in  the  concavity  formed  by  the  two  horns. 

6.  Close  to  the  gray  matter  in  the  anterior  basic  bundle. 

c.  In  the  posterior  columns  the  following  three  situations :    (1) 
Close  to  the  tip  of  the  posterior  horn.     (2)  A  small  area  between 
the  columns  of  Goll  and  Burdach  —  the  comma  tract.     (3)   Close  to 
the  posterior  fissure,  the  septomarginal  bundle. 

d.  Mingled  with  the  fibers  of  the  pyramidal  tract. 


CONDUCTING   FUNCTIONS   OP  THE   SPINAL    CORD 

The  Spinal  Tracts  —  Functionally  similar  nerve  fibers  within 
the  spinal  cord  are  seldom  isolated.  Almost  all  run  in  bundles  with 
other  fibers  serving  the  same  functions.  Practically  all  nerve  fibers 
within  the  cord  are  connected  by  means  of  collateral  fibers  with 
nerve  cells  of  more  than  one  segment.  These  collateral  fibers,  unlike 
their  parent  fibers,  which  are  medullated,  have  no  medullary  sheath. 
The  single  axis  cylinder  lies  embedded  in  a  layer  of  myelin  sur- 
rounded immediately  by  neuroglia. 

The  various  bundles  of  the  spinal  cord  may  be  divided  into :  (1) 
Proprio-spinal  or  internuncial  fibers,  fibers  connecting  various 
levels  of  the  spinal  cord,  some  of  which  are  ascending  and  others 
descending.  (2)  Ascending  bundles.  (3)  Descending  bundles. 

134 


THE  NERVOUS  SYSTEM 

Some  of  the  proprio-spinal  tracts  have  already  been  considered. 
Inasmuch  as  they  are  both  ascending  and  descending  we  will  de- 
scribe them  together  with  the  ascending  and  descending  tracts. 
(Figs.  61,  65  and  67.) 

Descending  Tracts  —  Pyramidal  Tracts  —  Found  immediately 
in  front  of  the  base  of  the  posterior  horns.  It  contains  fibers  which 
originate  in  the  nerve  cells  of  the  motor  area  of  the  cerebral  cortex 
and  run  without  interruption  through  the  cerebral  peduncles, 
through  the  pons  Varolii,  and  through  the  medulla,  where  they 
decussate  with  each  other  to  gain  the  opposite  side  to  enter  the 
pyramidal  tract  of  the  spinal  cord  and  terminate  by  a  terminal 
arborization  probably  directly  around  the  motor  cells  of  the  anterior 
horns  of  the  cord.  By  collaterals  they  communicate  with  the  cells 
of  several  levels. 

The  Prepyramidal  Tract  —  Situated  in  the  spinal  cord  immedi- 
ately in  front  of  the  pyramidal  tract.  It  begins  in  the  red  nucleus 
of  the  mid  brain.  The  tract  very  probably  represents  an  indirect 
cerebellar  spinal  tract,  in  other  words  it  continues  the  efferent  im- 
pulses from  the  cerebellum  through  the  superior  peduncles  through 
the  red  nucleus  to  the  spinal  cord. 

The  Vestibulo-Spinal  Tract  —  This  tract  establishes  connections 
between  the  higher  equilibrium  centers  and  the  spinal  cord.  It 
begins  in  the  cells  of  Deiter's  nucleus  of  the  medulla,  which  is  an 
important  substation  to  the  cerebellum.  The  fibers  of  the  vestibulo- 
spinal  tract  are  scattered  in  the  antero-lateral  column. 

The  Olivo-Spinal  and  Thalamo-Spinal  —  Situated  opposite  the 
tip  of  the  anterior  horn.  It  begins  in  the  optic  thalamus  and  in 
cells  of  the  inferior  olivary  body.  The  latter  may  be  regarded  as  a 
substation  for  many  of  the  fibers  between  the  thalamus  and  the 
cord. 

Tract  of  Marie  —  A  proprio-spinal  tract,  serving  the  same  pur- 
pose as  the  posterior  longitudinal  bundle  to  be  studied  later  in  the 
brain.  Its  fibers  are  both  ascending  and  descending  and  scattered 
in  the  anterior  columns. 

Comma  Tract  —  In  the  interval  between  the  columns  of  Burdach 
and  Goll,  chiefly  descending  branches  of  the  entering  posterior 
spinal  nerves. 

Septo-marginal  —  Chiefly  proprio-spinal,  and  situated  adjacent 
to  the  posterior  portion  of  the  posterior  fissure. 

136 


THE  NEEVOUS  SYSTEM 


THE  NERVOUS  SYSTEM 

Ascending  Tracts  —  Posterior  Columns  —  Divided  into  the  col- 
umn of  Burdach,  the  postero-lateral,  and  the  column  of  Goll  or 
postero-median.  The  fibers  of  the  posterior  columns  are  derived 
from  ascending  divisions  of  the  entering  posterior  nerve  roots.  As 
these  enter  they  occupy  first  the  postero-lateral  column  but  become 
displaced  internally  by  similar  fibers  entering  at  higher  levels. 
Therefore  the  column  of  Burdach  of  the  lumbar  region  becomes  the 
column  of  Goll  in  the  cervical  region.  In  the  cervical  region  the 
column  of  Goll  contains  fibers  from  the  lower  extremities  and  the 
column  of  Burdach  the  fibers  from  the  upper  extremity.  They 
terminate  in  the  medulla  around  cells  of  the  nucleus  cuneatus  and 
nucleus  gracilis. 

The  Direct  or  Dorsal  Cerebellar  Tract  —  Situated  just  anterior 
to  the  outer  extremities  of  the  posterior  horns,  external  to  the 
pyramidal  tract.  It  may  be  viewed  as  one  of  the  two  afferent  spino- 
cerebellar  tracts.  Its  fibers  originate  as  axons  of  the  cells  of  Clark 's 
column,  and  run  up  to  the  corpus  restiforme,  then  entering  the 
inferior  cerebellar  peduncle. 

The  Anterior  or  Ventral  Cerebellar  Tract  —  Situated  anterior 
to  the  dorsal  cerebellar  tract,  between  it  and  the  bundle  of  Helweg. 
It  ascends  through  the  medulla  to  join  the  superior  cerebellar 
peduncle,  by  which  it  enters  the  cerebellum,  to  end  in  the  cells  of 
the  ventral  nuclei  of  the  superior  worm.  Inasmuch  as  this  tract 
does  not  increase  in  size,  as  it  ascends,  some  of  its  fibers  may  join 
the  dorso-cerebellar  tract  or  end  in  the  cord. 

Spino-Thalamic  —  Situated  just  internally  to  the  anterior  cere- 
bellar fibers,  forming  a  tract  which  is  often  described  as  one  tract 
with  the  anterior  cerebellar  tract — the  tract  of  Gowers.  It  ter- 
minates in  the  cells  of  the  anterior  corpora  quadrigemina,  but 
chiefly  in  the  cells  of  the  optic  thalamus. 

Proprio- Spinal  Fibers  —  Chiefly  the  ascending  fibers  of  the 
tract  of  Marie  in  the  antero-lateral  column. 

Functions  of  the  Various  Tracts  —  Motor  impulses  descend 
through  the  pyramidal  tracts,  and  the  indirect  or  crossed  pyramidal 
tracts,  immediately  lateral  to  the  anterior  fissure. 

Impulses  of  pain  both  superficial  and  deep  enter  through  the 
posterior  roots,  cross  immediately  to  the  other  side  of  the  cord  and 
ascend  in  the  internal  portion  of  Gowers'  tract  to  the  optic  thala- 
inus.  (Fig.  68.) 

140 


THE  NERVOUS  SYSTEM 

Impulses  of  heat  and  cold  follow  the  same  course. 

Impulses  of  touch  and  pressure  cross,  after  running  upwards 
for  a  few  segments  of  the  cord,  to  the  other  side  of  the  cord  and 
ascend  to  the  optic  thalamus  in  the  antero-lateral  column.  Some  of 
the  fibers  of  cutaneous  touch  particularly  those  of  tactile  discrim- 


AO 


ALS 
Fig.  68. — The  course  of  the  fibres  composing  the  posterior  root. 

I.  The  fibres  of  the  posterior  columns. 

II.  Fibres  making  connection  with  Clark's  column  and  continued  upward 
in  the  posterior  cerebellar  tract,  P.  C. 

III.  IV,  and  V.    Fibres  forming  connection  with  posterior  horn  cells  and 
continued  upward  in  the  anterior  cerebellar  tract,  AC,  conveying  heterolat- 
eral  unconscious  afferent  impulses  of  muscular  coordination  and  reflex  tone. 
G.,  Gowers'  column  transmitting  impulses  of  pain,  heat  and  cold.     A.L.S., 
Antero-lateral    ascending    sensory    tract    conveying    impulses    of    touch    and 
pressure. 

ination  travel  upwards  uncrossed  in  the  posterior  columns.  Con- 
siderable evidence  exists  that  touch  impulses  occupy  a  different 
course  than  pain  and  temperature.  We  must  regard  superficial 
touch  sensations  as  compounded  of  superficial  tactile  discrimina- 
tion and  superficial  pressure  sensation.  In  the  disease  of  syringo- 
myelia,  the  senses  of  temperature  and  pain  are  affected  while  the 
sense  of  touch  is  not.  Moreover  unilateral  section  of  the  cord  does 

142 


THE  NERVOUS  SYSTEM 

not  completely  abolish  the  sense  of  touch  on  the  same  side  below  the 
level  of  the  lesion. 

Impulses  of  muscular  sensibility  may  be  divided  into  those 
which  reach  consciousness  and  those  which  do  not.  The  former  are 
all  homolateral  within  the  spinal  cord  and  make  up  the  posterior 
columns.  The  latter  are  partly  homolateral,  forming  the  direct  or 
posterior  cerebellar  tract,  and  partly  heterolateral,  forming  the 
anterior  cerebellar  tract. 

Unilateral  section  of  the  Cord  will  produce  the  following  symp- 
toms: 

Motor  paralysis  below  the  site  of  the  lesion  upon  the  same  side. 

Partial  loss  of  consciousness  of  the  position  of  the  limbs  below 
the  site  of  the  lesion  on  the  same  side. 

Complete  anesthesia  below  the  level  of  the  lesion  on  the  opposite 
side.  There  will  be  a  preservation  of  sensations  of  touch  and  pres- 
sure for  four  or  five  segments  below  the  level  of  the  lesion  on  the 
opposite  side.  There  will  be  slight  anesthesia  for  a  narrow  strip  at 
the  level  of  the  lesion  on  the  same  side.  This  narrow  zone  will  be 
above  a  hyperaesthetic  zone. 

There  will  be  a  paralysis  of  the  vaso-motor  nerves  below  the 
level  of  the  lesion  on  the  same  side.  Vasomotor  impulses  travel 
down  the  cord  from  the  medulla  on  the  same  side. 

SPINAL  FUNCTIONS 

For  the  study  of  the  functions  or  reactions  of  the  spinal  cord 
a  study  of  the  cord  separated  from  the  brain  furnishes  much  valu- 
able information.  We  are  then  able  to  study  what  may  be  termed 
pure  spinal  reactions,  reactions  uninfluenced  by  impulses  contin- 
ually descending  from  above.  An  animal  in  which  the  spinal  cord 
has  been  severed  from  the  brain  is  called  a  spinal  animal. 

Spinal  shock  is  the  first  effect  of  dividing  the  spinal  cord. 
There  is  a  great  fall  in  the  blood  pressure  and  absolute  paralysis 
of  the  skeletal  muscles  and  of  the  sphincters  and  abolition  of  all 
reflexes. 

The  shock  appears  to  only  exist  aboral  to  the  plane  of  section. 
In  monkeys,  for  instance,  after  section  of  the  cord  below  the  cer- 
vical region,  though  there  is  a  fall  of  blood  pressure  and  paralysis 
of  the  trunk  and  lower  extremities,  nevertheless  all  muscles  sup- 
plied by  nerves  issuing  from  the  cord  above  the  plane  of  section 

144 


THE  NERVOUS  SYSTEM 

are  active.  Immediately  after  the  section  the  animal  will  gaze 
out  of  the  window  in  a  contented  manner  and  even  catch  at  flies. 
After  a  period,  which  is  proportional  to  the  height  in  the  scale 
of  life  which  the  animal  occupies,  recovery  from  the  shock  occurs. 
Permanent  paralysis  of  voluntary  motion  and  loss  of  sensation 
remains  for  all  regions  below  the  plane  of  section. 


Fig.  69. — Tracing  of  the  flexion  of 
the  hip  in  the  "scratch-reflex." 
The  reflex  is  evoked  by  two  sepa- 
rate stimulations  (unipolar  faradiza- 
tion) at  points  ten  centimeters  apart 
on  the  skin  surface.  The  upper  sig- 
nal shows  the  time  of  application  of 
the  first  stimulation,  and  the  line 
immediately  below  that  the  frequency 
of  repetition  of  the  double  induction 
shocks  of  that  stimulation.  The  low- 
est line  signals  the  time  of  applica- 
tion of  the  second  stimulation;  the 
frequency  of  repetition  of  the  double 
shocks  in  this  stimulation  was  much 
greater  than  in  the  other  stimulation 
and  is  not  shown.  At  the  top  the 
time  is  marked  in  fifths  of  seconds. 
The  moment  of  commencement  of 
the  first  stimulation  is  marked  by  an 
abscissa  on  the  base  line.  The  pe- 
riods of  the  two  separate  stimula- 
tions overlap,  the  second  beginning 
a  full  second  before  the  first  ends, 
but  no  interruption  or  increase  of 
the  rate  of  rhythmic  reflex-response 
appears.  (Sherrington.) 


Recovery  from  the  Shock  —  From  the  shock,  however,  the 
animal  recovers.  The  blood  pressure  first  rises  to  normal.  The 
sphincters  become  functional  and  the  bladder  and  rectum  capable 
of  emptying  themselves.  Finally  muscular  tone  is  regained  and 
coordinated  movements  (reflexes)  may  be  excited  by  stimuli.  The 
first  reflexes  to  reappear  are  .those  dependent  upon  painful  or 
nocuous  stimuli  and  later  those  reflexes  depending  upon  stimula- 
tion of  nerve  endings  in  the  joints  and  end  organs  in  the  muscles. 

Cause  of  Spinal  Shock  —  Spinal  shock  does  not  depend  upon 

146 


THE  NERVOUS  SYSTEM 

the  lowered  blood  pressure  nor  to  the  trauma  of  the  operation. 
Regions  not  in  the  shock  above  the  plane  of  section  are  exposed 
to  the  same  lowered  blood  pressure  and  a  second  section  below  the 
first,  even  if  performed  with  little  precaution  to  avoid  trauma, 


Fig.  70. — Flexion-reflex.    Spinal  dog.    Latent  time  of  incremental  reflex  com- 
pared with  that  of  initial  reflex.    Unipolar  faradization  by  break  shocks. 

Kathode  at  needle-point  in  plantar  skin  of  outermost  digit. 
A  weak  initial  stimulus  is  delivered  and  maintained,  and  then  when  the 
resulting  reflex  movement  has  become  steady  the  stimulus  is  increased  in 
intensity  by  short-circuiting  5  ohms  from  the  primary  circuit.  The  rate  and 
intensity  of  the  break  shocks  are  marked  above  by  a  recording  electromagnet ; 
the  armature  is  arranged  to  have  an  ampler  excursion  when  the  current  is 
increased  at  the  point  marked  B.  The  latent  time  of  the  incremental  reflex  is 
seen  to  right  hand,  and  is  distinctly  shorter  than  that  of  the  initial  reflex. 
Time  below  is  written  in  1/100  sec.  and  in  seconds.  Abscissae  on  the  myograph 
line  show  the  moment  of  first  delivery  of  the  initial  (.4)  and  of  the  incre- 
mental (B)  stimuli.  (Sherrington.) 

does  not  add  to  the  degree  of  shock.  It  is  directly  dependent  upon 
the  cutting  off  of  the  normally  descending  impulses  from  higher 
parts  of  the  nervous  system  which,  so  to  speak,  keep  the  cord  awake 
and  responsive. 

Recovery  from  the  shock  depends  upon  the  power  of  the  spinal 
centers  to  acquire  a  more  independent  activity  of  their  own  in 

148 


THE  NERVOUS  SYSTEM 

the  absence  of  the  assistance  of  the  central  nervous  system.     In 
the  dog  recovery  may  even  occur  to  such  an  extent  that  it  may  be 

able  to  take  a  few  steps  if  it  be 
raised  and  given  a  push,  al- 
though it  cannot  walk. 

Swimming  movements  may 
be  carried  out  regularly.  They 
are  not  voluntary  but  purely  re- 
flex, the  necessary  sensory  stimu- 
lus being  supplied  by  the  exten- 
sion of  the  muscles,  and  are  of 
the  nature  of  alternate  flexion 
and  extension  in  the  hind  limbs 
when  the  animal  is  held  upright 
by  his  fore  limbs. 

A  Study  of  the  Spinal  Re- 
flexes —  Scratch  Reflex  —  Con- 
tinued gentle  stimulation  over 
the  shoulders  will  cause  rhyth- 
mic flexion  and  extension  of  the 
hind  limb  of  the  same  side  as 
though  making  an  attempt  to 
brush  off  the  irritant.  (Figs. 
69-71.) 

Sole  Reflex  —  Pricking  the 
sole  of  the  foot  will  cause  flex- 
ion of  the  leg  and  thigh  and,  if 
the  stimulus  is  strong  enough, 
extension  of  the  opposite  leg. 

Gentle  pressure  upwards 
against  the  sole  will  cause  ex- 
tension of  the  same  leg  and  flex- 


Fig.  71.— A,  B,  Scratch-reflex. 
The  tracings  show  the  usual  length- 
ening of  latency  on  reducing  the  in- 
tensity of  the  stimulation.  The  two 
tracings  are  in  reproduction  unequal- 
ly reduced,  but  the  frequency  of 
repetition  of  the  double-induction 
shocks  used  as  stimuli  was  the  same 
in  both  shocks  of  weaker  intensity  in 
B  than  in  A.  The  reflex  movement 
began  after  delivery  of  three  stimuli 
in  A,  after  delivery  of  nine  in  B. 
The  greater  intensity  of  the  stimuli 
in  A  is  also  evidenced  by  the  greater 
amplitude  of  the  movement  and  by 
the  longer  "after-discharge."  Time 
marked  in  fifths  of  seconds  below. 


ion  of  the  opposite  leg. 

Vascular  Reflex  —  A  rise  in 
blood  pressure  may  be  obtained 
from  afferent  stimulation  of  the 
digital  nerve.  (Fig.  72.) 

Bladder  and  rectal  reflex  is 
the  stimulus  within  the  bladder 


150 


THE  NERVOUS  SYSTEM 


and  rectum  of  urine  and  feces  which  will  cause  voluntary  evacua- 
tion of  these  organs.    Even  coitus  and  parturition  may  occur. 

Muscular  Tone  —  The  degree  with  which  muscular  tone  can 
return  is  illustrated  in  a  frog  which  has  recovered  from  spinal 
shock  by  section  of  its  posterior  spinal  nerves  on  one  side.  That 
side  will  then  be  perfectly  flaccid  and  completely  extended  con- 
trasting with  a  partially  flexed  position  of  the  other  leg  when  the 
animal  is  suspended.  (Fig.  73.)  In  other  words,  after  the  recovery 

from  the  shock  stim- 
uli are  continually  as- 
cending to  the  spinal 
cord  from  the  muscles 
themselves  which  ex- 
cite other  efferent 
stimuli  that  keep  the 
muscles  in  some  de- 
gree of  contraction. 
The  partial  contrac- 
tion, in  other  words, 
the  keeping  the  mus- 
cles in  a  condition  of 

Fig.  72.— Spinal  vasomotor  reflex;  dog;  300  days      wakefulness,  is  called 
after  spinal  transection  at  eighth  cervical  level; 


sp 

chloroform  and  curare.  Electrical  stimulation 
of  central  end  of  a  digital  nerve  of  hind  limb 
during  the  time  marked  by  signal  on  second 
line  from  bottom.  The  arterial  pressure  (caro- 
tid) rises  from  90  mm.  Hg.  to  208  mm.  Hg. 
Time  marked  below  in  2  seconds.  (Sherrington.) 


tone. 

Tendon  Reflexes  — 
The  patella  reflex  is 
only  one  of  various 
other  tendon  reflexes, 
indeed  the  phenome- 
It  illustrates  the  factors  determin- 


non  is  common  to  all  tendons, 
ing  muscular  tone. 

A  tendon  reflex  may  be  elicited  by  tapping  the  tendon  of  a 
limb  placed  in  a  flexed  condition  but  preferably  in  such  a  position 
that  the  tendon  is  somewhat  on  the  stretch.  For  the  right  patellar 
reflex  the  right  knee  should  cross  the  left  allowing  the  leg  to  hang 
loosely  over  the  left  one.  The  patellar  tendon  is  then  sharply 
struck.  Immediately  the  quadriceps  extension  of  the  right  limb 
will  contract,  causing  the  leg  to  give  a  little  jerk.  The  latent 
period  between  the  time  of  the  blow  to  the  tendon  and  the  con- 
traction is  very  short,  according  to  Gotch  only  .005  of  a  second, 

152 


THE  NERVOUS  SYSTEM 


which  is  also  the  duration  of  the  latent  period  when  the  vastus 
internus  is  directly  stimulated. 

Manifestly,  therefore,  it  must  bu  considered  possible  that  the 
cause  of  the  knee  jerk  is  the  direct  effect  of  tjie  blow  upon  the 
muscle,  but  this  cannot  be  all.  Section  of  the  posterior  nerve  roots 
of  the  third  and  fourth  lumbar  nerves  will 
abolish  the  knee  jerk,  so  also  stretching  the 
hamstring  muscles  or  the  antagonists  of  the 
flexors  or  weak  stimulation  of  the  nerve  sup- 
plying hamstrings. 

Section  of  the  hamstrings  or  their  nerve 
will  increase  the  knee  jerk  reflex.  These  facts 
indicate  that  an  afferent  path  to  the  spinal 
cord  is  necessary  for  the  elicitation  of  the  ten- 
don phenomenon,  and  also  a  relaxation  of  the 
antagonists  to  the.  muscles  which  produce  the 
contraction  of  the  muscles  which  are  con- 
cerned in  the  tendon  reflex  in  question. 

In  order  to  accomplish  this  relaxation, 
a  reflex  arc  through  the  spinal  centers  is  nec- 
essary, a  fact  also  attested  by  the  abolition  of 
the  knee  jerk  in  consequence  of  dividing  the 
posterior  spinal  roots. 

It  has  been  suggested  that  the  part  played 
by  the  spinal  cord  is  one  merely  maintain- 
ing muscular  tone,  keeping,  so  to  speak,  the 
muscles  in  a  state  of  wakefulness.  While  this 
suggestion  may  in  a  large  part  explain  the 
characters  of  the  tendon  reflex,  it  is  not  the 
entire  explanation.  More  accurate  measure- 
ments by  Jolly  of  the  current  of  action  in 
the  muscles  and  nerves,  with  a  sensitive  gal- 
vanometer, show  in  disagreement  with  the 
conclusions  of  Gotch  that  the  latent  period 
of  the  knee  jerk  contains  also  a  small  reduced 
reflex  time  of  .002  of  a  second,  corresponding  perhaps  with  only 
one  synapse.  These  measurements  are  as  follows: 


Fig.  73.— Illustrating 
the  difference  in  the 
tone  of  the  legs  of 
a  frog  when  the  pos- 
terior root  nerves 
of  one  side  have 
been  divided. 


154 


THE  NERVOUS  SYSTEM 

Time  of  knee  jerk 0053 

Time  in  afferent  ending 0004 

Time  in  nerve  conduction. 0014 

Time  in  motor  endings 0013     .0031 

.002  + 

Other  examples  of  tendon  reflexes  are  the  Babinski  sole  reflex 
and  Koernig's  sign. 

These  facts  are  all  confirmed  by  certain  pathological  conditions 
in  man. 

In  locomotor  ataxia,  in  which  there  is  degeneration  of  the 
posterior  columns  of  the  spinal  cord  containing  the  mechanism 
for  afferent  impulses  from  the  muscles,  there  is  a  loss  of  knee  jerk. 
In  the  disease  lateral  sclerosis,  in  which  the  pyramidal  tracts  are 
degenerated,  there  no  longer  exists  any  controlling  influence  from 
above  upon  the  motor  mechanism  of  the  cord  and  the  motor  mech- 
anism of  the  cord,  as  in  recovery  from  spinal  shock,  assumes  an 
independent  and  exaggerated  activity,  consequently  the  knee  jerks 
are  increased. 


SUMMARY  OF  THE  FUNCTIONS  OF  THE  SPINAL  CORD 

The  spinal  cord  must,  therefore,  be  viewed  as  a  part  of  a  mech- 
anism for  obtaining  a  certain  definite  coordinated  response  of  a 
limited  segmental  character  from  certain  peripheral  stimuli  and 
for  maintaining,  as  a  result  of  certain  deep  seated  stimuli,  a  mus- 
cular tone  in  the  skeletal  muscles.  It  also  maintains  a  tone  in 
certain  of  the  vessels  and  structures  within  the  body  cavities  con- 
veniently though  not  accurately  described  as  visceral  tone. 

The  maintenance  of  both  the  skeletal  tone  and  visceral  tone  is 
part  of  a  reflex  mechanism  and  is  absent  in  the  absence  of  the 
necessary  afferent  impulses  or  is  changed  with  a  change  in  the 
normal  relation  of  excitatory  or  inhibitory  impulses  from  above 
or  from  other  parts  of  the  nervous  system. 

The  Characteristics  of  a  Spinal  Reflex  —  Purpose-like  —  Every 
reflex  movement  may  be  described  as  a  purpose-like  movement  for 
the  simple  reason  that  every  reflex  is  an  action  frequently  used  by 
the  animal.  This  fact  constitutes  the  reason  why  during  the  process 

156 


THE  NERVOUS  SYSTEM 

of  development  the  various  neurons  are  so  connected  that  the 
various  reflexes  become  possible.  The  word  purpose-like  has  been 
used  instead  of  purposeful  because  every  reflex  act  is  at  the  same 
time  fateful. 


Fig.  74. — A:  The  "receptive  field,"  as  revealed  after  low  cervical  transection, 
a  saddleshaped  area  of  dorsal  skin,  whence  the  scratch-reflex  of  the  left 
hind  limb  can  be  evoked.  Ir  marks  the  position  of  the  last  rib. 

B:  Diagram  of  the  spinal  arcs  involved.  L,  receptive  or  afferent  nerve-path 
from  the  left  foot;  R,  receptive  nerve-path  from  the  opposite  foot;  Ra,  Rb, 
receptive  nerve-paths  from  hairs  in  the  dorsal  skin  of  the  left  side;  FC, 
the  final  common  path,  in  this  case  the  motor  neurone  to  a  flexor  muscle 
of  the  hip;  Pa,  Pb,  proprio-spinal  neurones.  (Sherrington.) 

Fateful  —  The  spinal  cord,  unless  inhibited  by  actions  of  the 
higher  nervous  system,  always  responds  in  a  definite  calculable 
manner.  The  beheaded  eel  will  wind  itself  around  a  red  hot  poker. 
The  beheaded  snake  will  spring  back  to  any  agent  grasping  its 
tail. 

158 


• 

t*     ac.  S  .2         i" 

C   O 

,2  a 


»  «8  H  G 


160 


THE  NERVOUS  SYSTEM 

Pleurisegmental —  All  reflex  movements  are  such  as  would 
under  usual  conditions  perform  useful  acts  for  the  animal,  and  all 
are  complicated,  involving  several  segments  of  the  spinal  cord. 
They  are  all  pleurisegmental  (see  Fig.  74). 

Capable  of  Spreading  —  The  plantar  or  sole  reflex  in  the  mam- 
mal not  only  involves  the  extremity  on  the  side  of  the  provoked 
sensory  stimulus  hut  may  also  involve  the  extremity  of  the  other 
side. 

The  spreading  of  the  response  is  always  in  a  definite  order 
called  irradiation,  and  just  what  response  is  called  forth  is  deter- 
mined hy  the  place  or  locus  of  the  peripheral  stimulus.  We  have 
thus  far  only  spoken  of  the  contraction  of  certain  sets  of  muscles 
in  the  description  of  this  response,  hut  the  contracting  muscles  are 
only  half  of  those  which  participate  in  the  reflex  act. 

Capable  of  Inhibition  —  No  reflex  can  effectively  take  place 
without  the  simultaneous  relaxation  of  the  set  of  muscles  antagon- 
istic to  those  which  are  undergoing  contraction.  During  the  re- 
flex withdrawal  of  one  foot  (flexion)  and  extension  of  the  oppo- 
site leg,  there  is  not  only  contraction  of  the  flexors  but  also  relaxa- 
tion of  the  extensors  on  the  one  side  and  contraction  of  the  exten- 
sors and  relaxation  of  the  flexors  on  the  other  side.  This  relaxa- 
tion is  accomplished  by  impulses  which  inhibit  centrally  the  im- 
pulses responsible  for  the  normal  muscular  tone  of  the  relaxing 
muscles.  The  relaxation  may  be 'measured  by  a  recording  instru- 
ment which  demonstrates  the  actual  lengthening  of  the  muscle. 

A  reflex  action  may  be  altogether  prevented  by  influences 
transpiring  in  other  portions  of  the  central  nervous  system.  Any 
inhibition  is  accomplished  by  nerve  fibers  running  to  the  central 
synapses  of  the  reflex  in  question  from  the  other  portions  of  the 
nervous  system.  Certain  reflexes  are  prepotent;  others  may  be 
made  prepotent  by  strong  stimuli.  Prepotent  reflexes  have  a 
tendency  to  inhibit  other  reflexes  which  may  be  transpiring. 
The  possibility  of  reinforcing  the  knee  jerk  reflex  by  pulling  apart 
the  clasped  hands  illustrates  the  presence,  normally,  of  constant 
inhibitory  impulses  to  the  extensors  of  the  thigh.  If  the  reinforc- 
ing act  precedes  the  stimulation  of  the  reflex  by  .06  sec.  the  inhibi- 
tion begins  to  appear. 

Capable  of  Prepotency  —  Certain  reflexes  possess  precedence 
over  others.  Reflexes  from  painful  or  nocuous  stimuli  will  proceed 

162 


THE  NERVOUS  SYSTEM 


in  the  place  of  other  reflexes  which  may  have  been  started.  Only 
one  reflex  can  proceed  at  a  time.  The  central  nervous  system  only 
attends  to  one  thing  at  a  time.  A  reflex  in  process  of  performance 
will  be  checked  by  another  reflex  started  by  a 
stronger  stimulus.  The  checking  process  is 
one  of  inhibition,  but  after  the  period  of  in- 
hibition has  passed  the  inhibited  reflex  will 
proceed  again  with  renewed  vigor,  as  though 
its  stimulus  during  the  period  of  inhibition 
were  really  effective  though  apparently  only 
accumulatively  so. 

Capable  of  Reenforcement  —  If  one  reflex 
is  proceeding,  another  reflex  giving  rise  to  an 
action  cooperating  toward  the  same  end  is 
started,  will  proceed  and  strengthen  the 
first  reflex.  Strongly  pulling  the  interlocked 
fists  apart  will  reinforce  the  knee  jerk. 

Capable  of  Fatigue  —  Too  frequent  excita- 
tion of  a  reflex  causes  fatigue  at  the  central 
synapse.  If  the  stimulus  exciting  a  reflex 
which  has  been  fatigued  is  moved  only  a  very 
slight  distance  to  the  side  of  the  locus  of 
the  reflex,  it  will  again  proceed  with  renewed 
vigor.  Therefore  the  most  important  site  of 
the  fatigue  must  lie  jn  the  synapse  upon  the 
afferent  side  of  the  reflex  arc. 

Explanation  of  the  "Mark  Time"  Move- 
ment —  Just  as  repetition  of  a  reflex  causes 
fatigue,  so  inhibition  causes  reenforcement  of 
a  reflex.  These  two  facts  explain  the  auto- 
matic swimming  movement  or  the  automatic 
and  continuous  flexion  and  extension  of  first 
one  leg  and  then  the  other  in  the  suspended 
dog.  The  weight  of  one  leg  produces  a  stimu- 
lus to  the  contraction  of  its  flexors,  accompanied  by  inhibition  of 
flexion  upon  the  opposite  side.  This  flexion  is  followed  by  slight 
fatigue  on  the  newly  flexed  side  and  a  dropping  of  the  inhibited 
side,  which  in  turn  is  stimulated  by  this  drop  or  extension  and, 
in  virtue  of  the  rest  during  the  previous  period  of  inhibition,  is 

164 


Fig.  75.  —  Scratch-re- 
flex interrupted  by 
a  brief  flexion-re- 
flex. 

The  time  of  appli- 
cation of  the  stimulus 
evoking  scratch-reflex 
is  shown  by  the  low- 
est signal  line ;  that  of 
the  stimulus  of  the 
flexion-reflex  in  the 
signal  line  immediate- 
ly above  the  other. 
Time  marked  in  fifths 
of  seconds  at  top  of 
the  record.  The 
scratch-reflex  returns 
with  increased  inten- 
sity after  the  inter- 
ruption. (Sherring- 
ton.) 


THE  NERVOUS  SYSTEM 


in  a  condition  to  respond  to  the  stimulus  produced  by  its  own 
extension.  This  stimulus  causes  it  to  be  flexed  and  the  flexion  in 
the  first  limb  to  be  inhibited  in  its  turn. 

In  the  excitation  and  control  of  reflex  actions  two  varieties  of 
afferent  impulses  are  concerned.    One  set  of  these  impulses  comes 


Fig.  75l/2. — Diagram  indicating  connections  and  actions  of  two  efferent  root- 
cells,  a  and  a'  in  regard  to  their  reflex  influence  on  the  extensor 

and   flexor   muscles   of   the   two   knees. 

a,  root-cell  afferent  from  skin  below  knee;  a',  root-cell  afferent  from  flexor 
muscle  of  knee,  i.e.,  in  hamstring  nerve;  e  and  e',  efferent  neurones  to  the 
extensor  muscles  of  the  knee,  left  and  right;  s  and  s',  efferent  neurones  to  the 
flexor  muscles;  E  and  E',  extensor  muscles;  F  and  F',  flexor  muscles.  The 
"schalt-zellen"  (v.  Monakow)  probably  between  the  afferent  and  efferent 
root-cells  are  for  simplicity  omitted.  The  sign  +  indicates  that  at  the 
synapse  which  it  marks  the  afferent  fibre  a  (and  a')  excites  the  motor  neu- 
rone to  discharging  activity,  whereas  the  sign  —  indicates  that  at  the  synapse 
which  it  marks  the  afferent  fibre  a  (and  a')  inhibits  the  discharging  activity 
of  the  motor  neurones.  The  effect  of  strychnine  and  of  tetanus  toxin  is  to 
convert  the  minus  sign  into  plus  sign.  (Sherrington.) 

166 


THE  NERVOUS  SYSTEM 

from  the  surface  and  may  be  termed  exogenous  afferent  impulses. 
The  second  set  originate  within  the  muscles  themselves  and  within 
the  tendons  and  joints.  They  are  the  deep  or  endogenous  impulses. 

The  first  set  are  chiefly  concerned  in  the  excitation  of  reflexes. 
The  second  set  chiefly,  though  by  no  means  exclusively,  with  the 
degree  with  which  the  response  to  any  particular  reflex  takes  place 
in  the  various  muscles  concerned. 

In  other  words  the  second  set  of  impulses  guide  or  control  the 
response  in  such  a  manner  that  it  is  possible  for  it  to  become  a 
coordinated  movement.  It  is  through  them  that  information  is  ob- 
tained as  to  the  degree  of  contraction  of  any  muscle. 

When  these  impulses  are  cut  off  the  position  of  the  limbs  be- 
comes abnormal.  It  is  due  to  this  fact  that  after  the  posterior  nerve 
roots  of  one  hind  limb  of  a  frog  have  been  divided  the  frog's  limb 
assumes  a  position  of  permanent  extension  and  will  hang  with  the 
legs  limp. 

If  all  the  posterior  nerve  roots  of  the  cervical  nerves  of  one  side, 
except  the  eighth,  of  a  monkey  are  divided  the  monkey  will  still 
use  its  arm  for  climbing,  but  the  movements  will  be  inexact.  The 
inexactness  is  chiefly  in  the  arm.  The  hand,  which  is  supplied  by 
the  eighth  cervical,  exercises  perfect  precision.  The  monkey  has 
lost  information  from  the  muscles  enabling  it  to  know  the  various 
degrees  of  contraction  of  the  muscles  of  the  arm.  If  the  eighth 
cervical  nerve  is  then  also  divided,  the  arm  will  become  totally 
paralyzed. 

Capable  of  Localization  —  With  the  same  certainty  that  a  defi- 
nite muscle  contracts  after  the  application  of  a  stimulus  to  its  motor 
nerve,  so  the  application  of  a  stimulus  to  the  peripheral  ter- 
mination of  a  sensory  nerve  will  call  forth,  in  the  absence  of  in- 
hibitory influences,  a  definite  response.  A  fixed  path  of  least  re- 
sistance for  the  propagation  of  the  impulse  to  and  through  and 
from  the  central  nervous  system  has  been  developed. 

Capable  of  Delay  —  No  response  to  a  stimulus  of  a  sensory 
neuron  is  immediate.  A  delay  exists  which  represents  the  time 
necessary  for  the  impulse  to  ascend  the  sensory  neuron  and  for  the 
impulse  to  pass  across  the  synapse  in  the  central  nervous  system 
and  for  the  terminally  provoked  impulse,  the  efferent  impulse,  to 
descend  the  motor  or  secretory  nerve. 

The  speed  of  impulse  along  nerve  fibers  is  known  and  the  latent 

168 


THE  NERVOUS  SYSTEM 

period  involved  in  the  starting  of  the  sensory  impulse  and  in  the 
production  of  the  effect  by  the  efferent  impulse,  after  the  latter 
has  reached  the  peripheral  termination  of  the  efferent  nerve,  are 
also  known.  When  the  time  taken  by  these  processes,  which  rep- 
resents all  the  time  of  that  portion  of  a  reflex  occurring  outside 
the  central  nervous  system,  is  subtracted  from  the  total  time  of 
the  simplest  unilateral  reflex  act,  there  will  be  left  over  .008  of  a 
second. 

This  time,  therefore,  represents  the  time  of  that  portion  of  a 
reflex  act  which  is  occupied  in  the  passage  of  its  impulse  across 
the  synapses  in  the  central  nervous  system.  It  is  called  the  reduced 
reflex  time.  When  the  reflex  is  a  crossed  reflex  the  reduced  reflex 
time  is  .004  of  a  second  longer.  Probably  two  additional  synapses 
are  involved  in  a  crossed  reflex,  so  that  the  time  occupied  in  a 
single  synapse  is  about  .002  of  a  second. 

Capable  of  Summation  —  To  produce  a  reflex  response  by  a 
single  stimulus  that  stimulus  must  possess  a  certain  strength.  A 
weaker  stimulus  is  a  subminimal  stimulus.  A  subminimal  stimulus, 
however,  is  not  necessarily  without  effect,  inasmuch  as  several, 
five  or  six,  subminimal  stimuli  applied  at  a  proper  interval  will 
result  in  a  response.  The  term  summation  is  applied  to  this  phe- 
nomenon. 

Capable  of  Block  —  Fatigue  illustrates  one  form  of  block, 
aamely,  an  increased  resistance  across  a  synapse.  The  existence 
of  a  definite  path  for  each  reflex  through  the  central  nervous  system 
demonstrates  the  presence  of  increased  resistance  to  that  reflex  in 
all  other  synapses  of  the  central  nervous  system.  The  reality  of  this 
increased  resistance  is  made  more  evident  when  it  is  dissipated  by 
the  administration  of  strychnine  or  the  tetanus  toxine. 

Capable  of  Facilitation  —  Some  resistance  to  the  passage  of  an 
impulse  across  the  central  synapse  of  a  reflex  exists  even  in  that 
synapse  which  belongs  peculiarly  to  the  reflex  in  question.  This 
resistance  can  be  measured  by  the  strength  of  current  necessary  to 
provoke  the  reflex.  It  may  be  diminished  by  frequently  provoking 
the  reflex  at  an  interval  not  short  enough  to  result  in  fatigue. 
This  diminution  of  resistance  across  a  synapse  by  use  is  called 
facilitation.  Upon  it  depends  the  possibility  of  education  and 
memory. 

The  Nature  of  the  Path  Across  a  Synapse  —  The  transmission 

170 


THE  NERVOUS  SYSTEM 

of  impulses  so  constantly  in  definite  paths  suggests  the  existence 
of  a  direct  connection  across  the  synapse.  Certain  observers  have 
found  good  evidence  of  such  direct  connections  among  some  of 
the  lower  orders  of  invertebrates  and  have  maintained  that  they 
exist  also  throughout  the  nervous  systems  of  -animals.  By  special 
staining  methods  they  have  attempted  to  demonstrate  actual  con- 
nections between  the  dendrites  of  a  nerve  cell  at  nodal  points  in 
the  terminal  arborization  of  the  afferent  nerve  fiber  to  this  cell. 
In  fact,  some  preparations  show  that  the  terminal  arborization 
around  a  central  nerve  cell  forms  an  actual  basket  of  a  netlike 
structure  closely  surrounding  the  cell  with  enlarged  nodal  points. 
Do  the  fibrillse  of  the  nerve  cell  run  out  into  the  dendrites  and 
connect  by  means  of  them  with  these  nodal  points?  The  indirect- 
ness of  such  a  path  through  a  synapse  may  account  for  certain 
phenomena,  such  as  delay  existing  at  a  synapse,  but  the  law  of 
forward  direction  will  ever  constitute  an  objection  to  the  existence 
of  a  direct  connection  across  a  synapse. 

There  must  exist  a  free  ending  to  the  dendrites  of  one  neuron, 
and  a  beginning  to  the  dendrites  of  the  neuron  next  in  the  chain, 
and  an  interval  filled  with  a  different  substance  between  the  two. 

Functions  of  the  Various  Portions  of  a  Reflex  Arc  —  The  func- 
tion of  the  nerve  fiber  is  solely  one  of  conduction.  We  may  exclude 
excitation. 

The  function  served  by  the  synapse  is  also  one  of  conduction. 

What,  however,  is  the  function  of  the  central  nerve  cell  ?  May 
it  originate  impulses,  or  does  it  modify  them,  or  does  it  solely 
conduct  them,  and  are  there  any  other  possible  functions  which  it 
may  perform? 

The  Function  of  the  Central  Nerve  Cell  —  In  answering  this 
question  all  the  vital  phenomena  presented  by  living  cells  must 
be  considered.  Foremost  among  these  is  the  maintenance  of  nutri- 
tion. We  have  seen  that  no  cell  is  capable  of  continued  existence 
without  a  nucleus.  The  sole  purpose  for  which  the  nervous  system 
has  developed  is  one  of  communication.  This  is  accomplished  by 
nerve  fibers.  Each  nerve  fiber  which,  in  many  instances^  consti- 
tutes the  major  part  of  the  neuron,  would  be  without  a  nucleus 
were  it  not  for  its  attachment  at  one  end  to  the  nerve  cell. 

Trophic — In  fact,  the  nerve  fiber  may  be  viewed  as  a  long  and 
permanent  pseudopod  of  a  nerve  cell,  and  without  attachment  to 

172 


THE  NERVOUS  SYSTEM 

the  cell  will  degenerate.  An  important  function,  therefore,  of 
the  nerve  cell  is  the  nutrition  of  its  nerve  fiber.  The  adjective 
trophic  is  the  term  which  is  used  to  describe  this  function. 

Transmission  —  Nerve  cells  must  transmit  impulses.  Less  cer- 
tainty, however,  attaches  itself  to  the  question  whether  nerve  cells 
modify  impulses  passing  through  them  apart  from  other  influences 
reaching  them  or  even  whether  they  may  be  spoken  of  apart  from 
the  fact  that  they  are  situated  at  a  synapse  as  switch  stations  of 
the  nerve  impulses. 

Automaticity  —  Much  evidence  exists  that  nerve  cells  do  not 
originate  nerve  impulses.  In  the  absence  of  all  afferent  stimuli 
they  become  functionless.  Metabolic  activity  with  the  evolution 
of  energy  transpires  within  them.  The  evidence  of  this  is  supplied 
by  the  rapid  loss  of  power  to  functionate  in  the  absence  of  oxygen. 

Clamping  the  aorta  soon  produces  a  paralysis  of  the  whole 
spinal  cord.  Nevertheless,  in  the  same  manner  a  lack  of  oxygen 
will  render  nerve  fibers  incapable  of  conduction. 

In  some  of  the  lower  invertebrates  the  ganglion  cells  of  afferent 
fibers  may  be  excised  without  injury  to  the  terminal  arborization 
of  the  afferent  and  efferent  nerve  fibers  to  these  cells  and  without 
immediately  influencing  in  any  way  the  function  of  the  concerned 
neurons.  So  far,  therefore,  as  the  nervous  activity  of  these  cells 
is  concerned  it  is  limited  entirely  to  transmission  and  trophic  func- 
tions. Like  other  cells  in  the  body  the  nerve  cells  have  become 
highly  differentiated  for  the  purpose  of  performing  most  effectively 
one  function. 

This  function  is  primarily  one  of  reaction,  a  reaction  which 
includes  two  factors,  excitability  and  conductivity.  It  is  very 
doubtful  whether  nerve  cells  possess  any  automatic  function  what- 
soever. Even  after  large  doses  of  strychnine  have  been  adminis- 
tered in  the  absence  of  all  afferent  impulses,  a  condition,  for  in- 
stance, which  exists  after  the  section  of  all  the  posterior  nerve 
roots,  a  frog  will  lie  absolutely  motionless. 

Such  serious  changes  in  respiration  are  induced  after  cutting 
off  all  afferent  impulses  that  it  is  possible  that  the  demarcation 
currents,  due  to  the  trauma  of  the  cut  nerves  may  account  for  the 
incomplete  and  deficient  respiration  remaining  after  such  an 
experiment. 

174 


THE  NERVOUS  SYSTEM 


TROPHIC   FUNCTIONS   OP   THE   CORD 

We  have  thus  far  considered  only  the  motor  and  sensory  func- 
tions of  the  spinal  cord.  The  spinal  cord  also  exercises  trophic 
functions  upon  both  the  muscles  and  the  skin.  When  its  connec- 
tions with  the  muscles  are  severed,  the  muscles  concerned  atrophy. 
When  the  skin  is  separated  by  division  of  the  posterior  nerve  roots 
it  also  shows  nutritional  changes.  The  skin  becomes  scaly,  glossy, 
and  the  hair  and  nails  show  changes.  When  certain  sensory  nerves 
become  inflamed  peculiar  eruptions  appear.  One  very  characteris- 
tic one  is  known  in  popular  language  as  "shingles."  The  paths 
exerting  trophic  functions  only  become  active  in  post-fetal  life. 
During  intrauterine  life,  even  in  complete  absence  of  the  nervous 
system,  the  muscles  develop  normally. 


176 


Ill 
THE  BRAIN 

Development  of  the  Brain  —  Phylogenetically  the  brain  is 
changed  anterior  segments  of  the  cerebrospinal  axis  having  devel- 
oped by  alterations  of  nervous  tissue  similar  in  every  way  to  the 
separate  segments  of  which  the  spinal  cord  in  mammal  is  composed 
and  of  which  the  more  definitely  segmental  nervous  system  of  the 
lower  animals  is  formed.  The  alterations  are  due  to  the  addition 
of  new  nerve  tracts  and  new  nerve  centers. 

The  new  nerve  tracts  connect  the  different  portions  of  the  brain 
and  the  brain  with  the  different  segments  of  the  spinal  cord.  The 
new  nerve  centers  serve  as  relay  stations  to  the  tracts  connected 
with  them,  making  possible  a  modification  of  the  afferent  impulses 
reaching  them,  by  passing  them  on  as  efferent  impulses  —  altered 
as  to  their  destination  or  strength  by  a  partial  switching  or  by  other 
impulses  also  reaching  these  centers  from  other  portions  of  the 
nervous  system  —  upon  other  afferent  tracts  to  these  same  centers. 

The  Three  Primary  Cerebral  Vesicles  —  After  the  primary 
neural  groove  has  been  transformed  into  a  tube  at  the  head  end, 
three  cavities  become  constricted  off  in  such  a  manner  as  to  par- 
tially separate  them.  These  three  cavities  are  the  three  primary 
cerebral  vesicles,  and  it  is  from  them  that  the  three  main  divisions 
of  the  adult  brain  develop. 

From  the  anterior  vesicle  develops  the  forebrain  or  the  prosen- 
cephalon.  It  may  be  divided  into  the  thalamencephalon,  including 
the  subsequent  cerebral  hemispheres,  the  lateral  ventricles,  the 
retina  and  the  olfactory  lobes,  and  the  diencephalon  which  includes 
the  third  ventricle  and  the  optic  thalami.  (Figs.  76-77.) 

From  the  middle  cerebral  vesicle,  or  the  mesencephalon,  develops 
the  corpora  quadrigemina  and  the  iter  of  Sylvius.  From  the  hind- 
brain,  the  rhomb encephalon,  develops  the  cerebellum,  the  pons  and 
the  upper  half  of  the  fourth  ventricle,  together  constituting  a  sub- 

178 


THE  NERVOUS  SYSTEM 


division  known  as  the  myelencephalon,  and  the  lower  half  of  the 
fourth  ventricle  known  as  the  metencephalon.- 

The  retina  of  the  eye  is  developed  from  two  lateral,  stalk-like 
protrusions  from  the  sides  of  the  primary  anterior  cerebral  vesicle. 


Corpora  quadrigemina. 


Cerebell 


Mesencephalon. 


Pineal  body. 


Pons 
varolii. 


Crura  cerebri.  Optic  thalamus.    |     Pituitary  body.  Foramen  of  Monro. 
Thalamencephalon. 

Fig.  76. — Diagrammatic  sagittal  section  of  a  vertebrate  brain.     (Morris.) 
4,  fourth  ventricle;  s,  cerebral  aqueduct;  3,  third  ventricle. 

Each  cerebral  hemisphere  also  develops  by  a  bud-like  expansion  of 
the  anterior  extremity  of  the  anterior  cerebral  vesicle.  The  bud 
contains  a  cavity  which  permanently  retains  its  connection  with  the 
original  cavity  of  the  anterior  cerebral  vesicle.  The  growth  of  these 

Epencephalon.        Optic  thalamus. 


Cerebellum 


Mesencephalon. 


Foramen  of  Monro. 


Fig.  77. — Diagrammatic  horizontal  section  of  a  vertebrate  brain. 
4,  fourth  ventricle;  3,  third  ventricle. 


Corpus 
striatum. 


(Morris.) 


buds  is  so  excessive  that  they  completely  cover  the  sides  and  dorsum 
of  the  rest  of  the  brain. 

The  original  cavity  of  the  buds  becomes  the  lateral  ventricle  and 
its  permanent  connection  with  the  cavity  of  the  anterior  cerebral 
vesicle  constitutes  what  afterwards  becomes  the  foramen  of  Monro. 

180 


THE  NERVOUS  SYSTEM 

g 

A  comparison  of  a  schematic  representation  of  the  primary  cerebral 
vesicles  and  the  adult  brain  will  make  these  facts  evident,  and  will 
clearly  establish  the  relative  positions  of  the  various  portions  of  the 
adult  brain. 

Beginning  at  the  transition  between  the  spinal  cord  and  the 
brain  and  passing  from  below  upwards,  the  central  canal  of  the 
spinal  column  opens  out  into  the  fourth  ventricle  of  the  brain.  In- 
cluding its  pontine  portion  the  latter  measures  nearly  two  inches  in 
length  and  about  three-quarters  of  an  inch  in  breadth  at  its  widest 
portion.  Posterior  to  the  medulla  and  forming  a  large  portion  of 
the  roof  of  the  fourth  ventricle  is  the  cerebellum. 

The  cerebellum  forms  a  large  and  separate  division  of  the  human 
brain.  It  measures  about  four  inches  by  two  by  one  and  a  half. 
The  most  prominent  structures  forming  the  lateral  boundaries  of 
the  fourth  ventricle  are  the  superior,  middle  and  inferior  peduncles 
of  the  cerebellum.  These  are  thick  bundles  of  nerve  fibers  by  which 
the  cerebellum  is  connected  with  the  mid-brain,  the  medulla  and  the 
spinal  cord  respectively. 

The  Fourth  Ventricle  —  The  floor  of  the  fourth  ventricle  con- 
sists of  gray  matter,  which,  in  this  portion  of  the  brain,  represents 
the  gray  matter  of  the  spinal  cord  displaced  posteriorly  by  the 
opening  of  the  central  canal  of  the  latter. 

It  is  diamond-shaped  and  divided  at  its  middle  by  transversely 
running  strands  of  nerve  fibers,  the  strice  acusticce,  into  an  upper 
pontine  and  a  lower  bulbar  portion.  On  the  floor  of  the  bulbar 
portion  is  a  triangular  depression,  the  ala  cinerea,  separating  a 
lateral  triangular  prominence,  the  tuberculum  acusticumf  from  a 
median  prominence,  the  trigonum  hypoglossi.  (Fig.  78.) 

The  nucleus  of  the  pneumogastric  nerve  forms  the  gray  matter 
of  the  ala  cinerea.  External  to  it  is  the  nucleus  of  the  eighth  nerve. 
It  overlies  the  position  of  the  more  deeply  placed  Deiters'  nucleus, 
and  extends  up  under  the  striae  acusticae  into  the  floor  of  the  pon- 
tine portion  of  the  fourth  ventricle.  In  this  region  it  is  separated 
by  a  shallow  depression  from  a  more  medially  placed  elongated 
prominence,  the  eminentia  teres.  The  eminentia  teres  is  formed  by 
the  gray  matter  of  the  nucleus  of  the  sixth  nerve.  It  corresponds 
in  its  position  above  the  striae  acusticae  to  the  trigonum  hypoglossi 
below,  and  its  gray  matter  is  the  direct  continuation  of  the  gray 
matter  of  the  latter. 

182 


THE  NERVOUS  SYSTEM 


184 


THE  NERVOUS  SYSTEM 

The  Iter  of  Sylvius  —  Passing  further  upwards  in  the  exam- 
ination of  the  brain,  the  cavity  of  the  fourth  ventricle  becomes 
again  contracted  into  a  narrow  canal,  the  iter  of  Sylvius  or  the 
Sylvian  aqueduct,  which  runs  between  it  and  the  third  ventricle. 
It  is  rather  more  than  half  an  inch  long.  Like  the  central  canal  of 
the  spinal  cord  it  is  surrounded  by  gray  matter.  The  gray  matter 
of  its  floor  gives  rise  to  the  fourth  and  third  nerves. 

Four  large  nuclei,  two  upon  each  side  of  the  middle  line,  cover 
its  roof.  These  are  called  the  superior  and  inferior  corpora  quad- 
rigemina.  Each  forms  a  prominent  rounded  eminence  upon  the  roof 
of  the  Sylvian  aqueduct.  The  superior  is  intimately  related  with  a 
smaller  cylindrical  eminence  passing  in  an  external  direction  from 
it.  It  is  called  the  external  geniculate  body  and  receives,  together 
with  the  superior  corpora  quadrigemina,  a  large  number  of  the 
fibers  of  the  optic  nerve.  A  similar  cylindrical  eminence  passes 
outward  from  the  inferior  corpora  quadrigemina.  It  is  called  the 
internal  geniculate  body,  and  receives  with  the  inferior  corpora 
quadrigemina  fibers  from  tracts  originating  in  connection  with  the 
nucleus  of  the  auditory  nerve,  situated  below  the  medulla. 

The  Third  Ventricle  —  At  its  upper  extremity  the  iter  of  Syl- 
vius enters  the  third  ventricle.  This  is  a  narrow,  cleft-like  cavity, 
contained  between  two  large  nuclei  of  gray  matter  called  the  optic 
thalami. 

The  optic  thalami  form  the  most  important  and  last  subrelay 
station  for  many  tracts  between  the  cerebrum  and  lower  portions 
of  the  brain  or  spinal  cord. 

The  third  ventricle  is  roofed  in  by  the  concave  lower  surface  of 
the  corpus  callosum,  a  large  mass  of  nerve  fibers  which  collected  in 
an  elongated,  flattened  bundle  connects  the  two  cerebral  hemi- 
spheres. It  is  curved  in  such  a  manner  to  be  convex  above  and 
concave  below. 

The  floor  of  the  third  ventricle  is  formed  of  the  following 
structures  beginning  at  the  front:  The  anterior  perforated  space, 
a  flat  plane  of  gray  matter  which,  with  the  infundibulum,  forms  a 
funnel-like  cavity,  leading  down  to  the  stalk  of  the  pituitary  gland, 
a  pea-sized  structure  situated  below  and  between  the  two. 

Behind  the  infundibulum  is  another  flattened  plane,  the  tuber 
cinereum.  More  posterior  are  two  knob-like  structures,  one  on  each 

186 


THE  NERVOUS  SYSTEM 

side  of  the  middle  line,  called  the  corpora  mammillaria  or  corpora 
albicans. 

Between  these  and  the  anterior  opening  of  the  Sylvian  aque- 


Fig.  79. — Under-surface  of  a  simply  convoluted  European  brain.     (Quain.) 
Sulci — orb.,  orbital  (sagittal  rami) ;  o.  tr.,  transverse  orbital;  olj.,  olfactory; 
ti,  tz,  tz,  first,  second,  and  third  temporal;  coll.,  collateral  (fourth  temporal); 
cole.,  calcarine. 

Gyri— R,  gyrus  rectus;  Tlf  Ts,  T*,  T5,  first,  third,  fourth,  and  fifth  tem- 
poral; H,  hippocampal;  s.r.a.,  caput  gyri  hippocampi;  unc.,  uncus. 

ch.,  chiasma;  s.p.a.,  substantia  perforata  antica;  i.e.,  tuber  cinereum;  m, 
corpora  mammillaria,  accidentally  separated  from  one  another  in  the  prepara- 
tion; cr.,  crusta;  tm,  tegmentum;  spl.f  splenium  of  callosum. 

duct,  or  iter  of  Sylvius,  is  another  inclined,  flattened  plane  of  gray 
matter,  the  posterior  perforated  space.  Immediately  above  the  an- 
terior opening  of  the  iter  of  Sylvius  is  the  posterior  commissure, 
consisting  of  a  band  of  white  fibers  running  across  the  two  sides  of 

188 


THE  NERVOUS  SYSTEM 

the  brain  in  this  situation  and  composed  of  commissural  fibers  con- 
necting the  posterior  termination  of  the  visual  tracts.  The  extrem- 
ities of  the  commissure  are  in  close  relation  to  the  superior  corpora 
quadrigemina. 

Above  this  commissure  are  the  two  stalks  of  the  pineal  gland, 
which  rests  upon  a  flattened  triangular  surface  in  front  of  and 


Genu    of   corpus   callosum. 
Cingulate 


Body  of  corpus  callosu 
Intermediate  mass. 
Fornix.  1    I 
Septum    pellucidum. 
Marginal    gyrus.,   JUJ-Jgssj 


fissure 


Tela    choroklea    ventriculi    tertll. 
Cingulate  gyrus. 
Callosal    fissure. 

Splenium  of   corpus  callosum. 
Paraeentral    lobule. 
<VntraI    tissure. 

Subparletal    fissure. 
Precuneate   lobnlo 
ii- 

tissure. 
Calcarine 
fissure. 
Cuneate 
lobule 


Lamina   terminalis. 

Optic  recess. 

Optic  nerve. 

Optic   commissure. 

Hypophysis. 

Anterior   commissure. 
Foramen  of  Monro. 

Third    nerve. 
Corpus  mammillare. 
Third  ventricle. 
Cerebral    peduncle. 
Pons. 
Suprapineal    recess. 

Pineal  body. 
Cerebral   aqueduct. 


Cerebellum. 
Medulla  oblongata. 
Fourth    ventricle. 
Superior  medullary  velum. 
Corpora  quadrigemina. 


Fig.  80. — Median  section  of  an  adult  brain.     (Quain.) 


between  the  two  superior  corpora  quadrigemina.     This  surface  is 
called  the  trigonum  habenulce. 

Two  other  commissures  cross  the,  third  ventricle,  the  middle  and 
anterior  commissure.  The  former  connects  the  two  optic  thalami, 
and  the  latter  is  situated  at  the  extreme  anterior  end  of  the  third 
ventricle  at  the  upper  extremity  of  the  anterior  perforated  space, 
and  is  in  relation  with  it.  It  contains  commissural  fibers  of  the 
olfactory  system.  One  more  bilateral  band  of  commissural  fibers, 
the  pillars  of  the  fornix  and  the  fornix  itself,  appears  in  the  third 

190 


THE  NERVOUS  SYSTEM 

ventricle.  They  run  in  an  anteroposterior  direction.  They  emerge 
from  the  corpora  albicans,  run  forward  to  curve  in  front  of  the 
foramen  of  Monro,  and  then,  reaching  the  roof  of  the  third  ven- 
tricle, they  run  backwards  upon  the  under  surface  of  the  corpus 
callosum,  diverging  as  they  pass  backwards  so  that  ultimately  they 
acquire  a  position  a  little  external  to  the  third  ventricle,  appearing 
by  their  external  edge  in  the  cavity  of  the  lateral  ventricle  upon 
the  upper  and  posterior  surface  of  the  optic  thalami.  In  this  situa- 
tion on  the  side  of  the  lateral  ventricle  they  are  overlapped  by  a 
vascular  fold  of  the  ependyma  or  remnants  of  the  layer  of  epithe- 
lium which  originally  forms  the  roof  of  the  third  ventricle,  before 
the  latter  with  its  ependymal  roof  is  covered  over  by  the  back- 
wardly  growing  hemispheres.  The  ependyma  in  this  manner  be- 
comes enclosed  between  the  fore-  and  mid-brain  and  to  some  extent 
inverted  laterally  into  the  cavity  of  the  lateral  ventricle  by  its  rich 
supply  of  blood  vessels.  It  is  appropriately  named  in  this  situation 
the  velum  inter positum. 


THE    CEREBRAL    HEMISPHERES 

Their  Development  —  The  cerebral  hemispheres  are  formed  by 
an  enormous  growth,  at  first  forwards  then  upwards  and  finally 
backwards,  of  the  anterior  extremity  of  the  anterior  cerebral  ves- 
icles. The  optic  thalami  develop  from  the  thickenings  of  the  lateral 
walls  of  the  anterior  cerebral  vesicle  and,  therefore,  belong  essen- 
tially to  the  third  ventricle. 

The  Lateral  Ventricles  —  Inasmuch,  however,  as  the  cerebral 
hemispheres  grow  from  the  anterior  end  of  the  anterior  cerebral 
vesicle,  preserving  within  them  a  continuation  of  the  cavity  of  the 
anterior  cerebral  vesicle,  they  may  be  appropriately  viewed  as  rep- 
resenting a  pre-cerebral  vesicle,  developed  in  front  of  the  anterior 
cerebral  vesicle.  This  so-called  pre-cerebral  vesicle  is  carried  back 
with  the  backwardly  growing  cerebral  hemispheres,  and  is  pre- 
served in  the  adult  brain  as  the  lateral  ventricles. 

The  Foramen  of  Monro  —  As  the  lateral  ventricle  is  carried 
backward  external  to  the  third  ventricle,  its  connection  with  the 
third  ventricle  is  placed  laterally  on  each  side  in  the  anterior  ex- 
tremity of  the  third  ventricle.  The  passage  of  connection  is  called 
the  foramen  of  Monro. 

192 


THE  NERVOUS  SYSTEM 


194 


THE  NERVOUS  SYSTEM 

The  Body  —  The  lateral  ventricle  of  the  brain  possesses  a  body 
and  three  horns.  (Figs.  82  and  83.) 

The  body  is  roofed  over  by  the  corpus  callosum.  Its  inner  wall 
is  bounded  by  the  fornix,  overlapped  from  below  by  the  edge  of  the 


Fig.  82. — View  of  the  lateral  ventricle  from  above.  Natural  size.  (Quain.) 
The  preparation  was  made  with  the  brain  in  situ  (hardened).  The  skull 
cap  and  membranes  having  been  removed,  the  brain  was  sliced  down  to  the 
level  of  the  corpus  callosum.  The  left  lateral  ventricle  was  then  opened  by 
cutting  away  its  roof,  and  the  island  exposed  by  slicing  away  the  opercula. 
The  drawing  is  made  from  a  photograph. 

/. R.,  insula  Reilii  (the  line  points  to  the  middle  of  the  three  gyri  breves); 
s.c.,  sulcus  centralis  insulae;  g.L,  gyrus  longus  insulse;  c.c.,  corpus  callosum; 
n.L.,  nerves  of  Lancisi;  str.  t.,  stria  tecta;  f.mi.,  forceps  minor;  /.ma.,  forceps 
major;  c. av  cqrnu  anterius  of  ventricle;  c.p.,  cornu  posterius;  c.i.,  entrance 
to  cornu  inferius;  /.  M .,  foramen  Monroi;  s.M.,  sulcus  leading  backwards 
to  the  foramen  Monroi;  c.str.,  nucleus  caudatus  of  corpus  striatum;  th.opt., 
thalamus,  anterior  tubercle;  pl.ch.,  plexus  choroides;  /.,  fornix;  /',  its  column; 
h.t  posterior  end  of  hippocampus;  tri.,  trigonum  ventriculi;  calcar,  calcar 

196 


THE  NERVOUS  SYSTEM 

velum  interpositum,  called  here  the  ckoroid  plexus,  because  of  the 
vascular  folds  present  in  its  edge.  Its  floor,  which  curves  upward 
externally  to  meet  the  roof,  is  formed  from  within  outward  by  the 


Fig.  83. — View  from  above  and  the  side  of  the  whole  left  lateral  ventricle. 
Natural  size.     (Quain.) 

This  is  a  further  dissection  of  the  preparation  shown  in  Fig.  82.  The 
insula  has  been  sliced  away  and  the  inferior  cornu,  c.  i.,  exposed.  Within 
this  are  seen  the  following  parts:  fi.,  fimbria,  continued  from  the  fornix;  h., 
the  hippocampus;  coll.,  the  eminentia  collateralis.  The  other  lettering  as  in 
Fig.  82. 


upper  surface  of  the  optic  thalamus,  the  tcenia  semicircularis,  a 
band  of  white  fibers  extending  from  the  region  of  the  septum  luci- 
dum  in  front  backward  and  outwards  along  the  external  superior 
border  of  the  optic  thalamus,  between  it  and  a  large  elongated 

198 


THE  NERVOUS  SYSTEM 


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THE  NERVOUS  SYSTEM 

nucleus  of  gray  matter  immediately  external  to  it  and  called  the 
caudate  nucleus. 

The  Caudate  Nucleus  —  The  caudate  nucleus  is  a  mass  of  gray 
matter  which  really  develops  in  the  external,  downwardly  curving 
fibers  of  the  corpus  callosum.  It  follows,  therefore,  the  general 
shape  of  the  curve  of  the  concave  lower  surface  of  this  body  and, 
as  well,  the  curve  of  optic  thalami,  from  the  outer  border  of  which 
it  lies  separated  by  the  taenia  semicircularis. 

From  within  outwards  the  optic  thalami,  the  taenia  semicircu- 
laris, and  caudate  nucleus  form  the  floor  of  the  body  of  the  lateral 
ventricle  and  the  roof  of  its  inferior  horns. 

The  Anterior  Horns  —  The  anterior  horns  of  the  two  lateral 
ventricles  of  the  brain,  arching  around  the  anterior  extremity  of 
the  optic  thalami,  are  separated  from  each  other  by  the  septum 
lucidum,  which  contains  the  fifth  ventricle. 

The  Posterior  and  Inferior  Horns  —  Only  the  space  common  to 
both  the  inferior  and  posterior  horns  bounds  the  posterior  extrem- 
ity of  the  optic  thalami.  The  floor  of  this  common  region  is  formed 
by  a  rather  large,  discoid  eminence  called  the  trigonum  ventriculi. 
Beginning  at  an  area  anterior  to  this  eminence  and  therefore  be- 
tween it  and  the  posterior  extremity  of  the  optic  thalamus  and 
extending  to  the  tip  of  the  inferior  horn  along  its  inner  wall,  is  an 
elongated,  rounded  eminence  called  the  hippocampus  major.  It 
ends  anteriorly  in  a  club-like  extremity  resembling  an  animal's 
paw.  Behind  the  posterior  internal  aspect  of  the  trigonum  ven- 
triculi and,  therefore,  forming  the  internal  wall  of  the  posterior 
horn,  is  another  elongated,  rounded  eminence,  the  hippocampus 
minor.  Another  elongated  fold  or  ridge  appearing  in  the  inner 
wall  of  the  posterior  horn  is  the  calcar  avis.  It  corresponds  to  an 
important  fissure  on  the  internal  surface  of  the  brain.  Above  it  in 
the  angle  of  the  inner  wall  and  roof  of  the  posterior  horn  is  a  fold 
caused  by  the  fibers  of  the  corpus  callosum,  running  to  the  occipital 
lobe  of  the  brain.  It  is  called  the  forceps  major. 

The  Hemispheres  —  The  external  surface  of  the  brain  presents 
certain  important  fissures  separating  convolutions  identified  with 
various  nervous  activities. 

The  Sylvian  Fissure  —  Upon  the  external  surface  is  the  Sylvian 
fissure.  A  deep  fissure  running  horizontally  backwards  from  a 
position  corresponding  to  the  posterior  border  of  the  lesser  wing  of 

204 


THE  NERVOUS  SYSTEM 


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THE  NERVOUS  SYSTEM 


Great  longitudinal  fissure  of  the  cerebrum — Fissure 
longitudinalis   cerebri. 

Frontal  pole — Polus  frontalis 

Olfactory  sulcus — Sulcus  olfactorius.  \ 
Orbital  sulcl— Sulci  orbitales. 


Orbital  gyri 


Temporal     pole  —  Polus 
temporalis. 


Trigonum  olfactorlum. -v 


Optic  commissure 
or  chiasma  — 
Gbiasma  opticum. 

Uncus  or  hook  of 
the  hippocampal 
gyrus. 

Entrance  to  the 
choroidal  fissure. 


Collateral  fissure 
— Fissura  col- 
lateralis. 


Third  (or  Inferior) 
temporal  sulcus — 
Sulcus  tempora'lis 
inferior. 


Third    temporal  gyrus 
Isthmus  of  the  gyrus  \ 
nicatus  —  Isthmus     gyri 
fornicata. 


Olfactory  bulb — Bulbus  olfactorius. 

Olfactory  tract — Tractus  olfactorius. 

Gyrus  rectus,  or  straight  gyrus. 

Root  of  the  olfactory  tract,   inner 
or  mesial,  middle  or  gray  and 
outer    or   lateral    root — Striae 
olfactoriae,      medialis,      inter- 
media lateralis. 
Anterior  perforated  space 
— Substantia     perforata 
anterior. 

x  Limen  Insulse,  or  thresh- 
old   of    the    island. 
Fissure      of      Sylvius — 
Fissura   cerebri     lat- 
eralis    (Sylvii). 

Nucleus  amygdalae, 
or  amygdaloid  nu- 
cleus. 

Cerebral  peduncle  or 
crus  cerebri 
(crusta)  —  Pedun- 
culus  cerebri 
(based  pedunculi). 

Posterior  perforated 
space  or  fossa  of 
Tarini — Substantia 
perforate  posterior. 


Substantia  nigra. 


Tegmentum. 


3/4 


Fourth  temporal  gyrus. 


Aqueduct  of  Sylvius — 

Aqueeductus  cerebri  (Sylvii). 
Quadrigeminal    lamina — Lamina 
quadrigemina. 

Splenium  of  the  corpus  callosum — Splenium 

Gyrus  fornicatus — Gyrus  fornicatus.  j        i  corporis   callosi. 

Occipital  pole — Pole  occipitalis.    Fifth   temporal  gyrus.  Hippocampal  gyrus. 

Great    longitudinal    fissure    of    the    cerebrum — Fissure  longitudinalis    cerebri. 

Fig.  94. — The  inferior  or  basal  surface  of  the  cerebrum,  facies  basalis  cerebri;  the  whc 
extent  of  this  surface  is  visible,  the  medulla  oblongata,  pons  varolii,  and  cerebellum 
(i.e.,  the  rhombencephalon)  having  been  removed  by  a  transverse  section  through  the1 
mid-brain.  Convolutions  and  furrows  of  the  hemispheres,  gyri  et  sulci  cerebri.  The 
frontal,  temporal,  and  occipital  poles  of  the  hemispheres. 

The  anterior  extremity  of  the  left  temporal  lobe  has  been  cut  away,  the  optic  com- 1 
missure   or   chiasma   has  been   cut  through  in  the  median  plane,  and  its  left  half  has  been    . 
removed.     The   anterior  perforated  space   has  thus  been  fully  exposed   on   the  left  side, 
and  its  relations  to  -the  threshold  of  the  island,    limen   insulse,   and   to   the   parts    of   the 
rhinencephalon   situate    on   the    mesial   surface  of  the  hemisphere,  have  been  made  mani-  I 
fest.     The  olfactory  tract,  tractus  olfactorius,  has  been  cut  away  on  the  right  side,  in  order 
to  display  the  olfactory  sulcus.     (Toldt.) 


222 


THE  NERVOUS  SYSTEM 

the  sphenoid  to  terminate  in  a  posterior,  upturned  extremity  in  the 
center  of  the  parietal  lobe.  It  separates  the  frontal  lobes  and  an- 
terior portion  of  the  parietal  lobes  from  the  temporal  lobes. 

The  Fissure  of  Rolando  —  The  Fissure  of  Rolando,  beginning 
at  a  point  corresponding  on  the  external  surface  of  the  skull  to  .55 
of  the  distance  from  the  frontal  prominence  to  the  occipital  tuber- 
cle, it  runs  downwards  and  forwards  at  an  angle  of  67%°  until  it 
nearly  reaches  the  fissure  of  Sylvius.  It  separates  the  frontal  from 
the  parietal  lobes. 

Superior  and  Inferior  Precentral  and  Intraparietal  Fissures  — 
Fissures  parallel  to  the  fissure  of  Eolando,  the  superior  and  inferior 
precentral  fissures  in  front,  and  the  intraparietal  fissure  behind, 
separate  the  ascending  frontal  convolution  from  the  rest  of  the 
frontal  lobe  and  the  ascending  parietal  convolution  from  the  rest 
of  the  parietal  lobe.  The  remainder  of  the  external  surface  of  the 
frontal  lobe  is  composed  of  the  superior  middle  and  inferior,  or 
first,  second  and  third  frontal  convolutions. 

Parietal  Lobes  —  The  remainder  of  the  parietal  lobe  is  com- 
posed of  the  superior  parietal  lobe,  contained  between  the  forks  of 
the  upper  extremity  of  the  intraparietal  fissure;  the  supra-marginal 
convolution,  curving  around  the  posterior  upper  extremity  of  the 
fissure  of  Sylvius,  and  the  angular  convolution  which  curves  around 
the  posterior  extremity  of  the  superior  temporal  fissure. 

Superior  Temporal  Fissure  —  The  superior  temporal  fissure 
runs  below  and  parallel  to  the  fissure  of  Sylvius  and  separates  the 
first  or  superior  temporal  convolution  from  the  second  or  middle 
temporal  convolution. 

The  Boundaries  of  the  Occipital  Lobe  —  Posterior  to  the  parie- 
tal and  superior,  middle  and  inferior  temporal  convolutions  is  the 
occipital  lobe.  It  is  separated  from  these  lobes  on  the  external  sur- 
face of  the  hemisphere  by  an  imaginary  line  drawn  from  the  point 
where  the  occipito-parietal  fissure  appears  on  the  external  surface 
of  the  brain  and  the  pre-occipital  notch.  The  last  is  an  indentation 
on  the  brain  produced  by  the  attachment  of  the  anterior  border  of 
the  tentorium  cerebelli.  The  frontal,  parietal,  occipital  and  tem- 
poral lobes  extend  over  upon  the  internal  surface  of  the  brain. 

The  Calloso-marginal  Fissure  —  The  inferior  limit  of  the  fron- 
tal lobe  on  the  internal  surface  of  the  hemisphere  is  founded  by  the 
c  all  o  so -marginal  fissure.  This  fissure  is  a  prominent  fissure  run- 

224 


THE  NERVOUS  SYSTEM 


ning  concentric  with  the  corpus  callosum  about  half  way  between 
the  latter  and  the  free  margin  of  the  internal  surface  of  the  hemi- 
sphere. Its  posterior  extremity  turns  upward  to  the  free  margin 
of  the  internal  surface  of  the  hemisphere  in  the  parietal  lobe  to  a 
point  posterior  to  the  fissure  of  Rolando. 

The  Limbic  Lobe  —  The  calloso-marginal  fissure  separates  the 
frontal  lobe  from  the  falciform  or  cingulate  or  limbic  lobe.    All  of 


S.  precentralls  meslalis 

S.  centralis   (Roland!) . 
Pars  marginalis  s 

cinguli. 

S.  parietalis 

superior 

S.  parieto- 
occipitalis. 


S.  cinguli. 


S.  corporis  callosi. 


__  rostralis. 

Incisura  temporalis. 


calcarinus. 

S.  subparietalis. 

S.  collaterals. 


Fascia  dentata. 


collateralis. 
temporalis  inferior. 


Fig.  95. — Left   cerebral  hemisphere  from  the  mesial  aspect.     Natural   size. 

(Quain.) 

The  label  "caput  hippocampi"  has  been  placed  too  far  forwards.    The  caput 
hippocampi  does  not  extend  in  front  of  the  incisura  temporalis. 

these  names  are  given  to  the  convolutions  below  the  calloso-mar- 
ginal fissure.  They  are  concentric  with  the  corpus  callosum  and 
curve  around  its  anterior  and  posterior  extremity.  Below  the  pos- 
terior extremity  of  the  corpus  callosum  it  becomes  connected  by  a 
narrow  constricted  portion  with  an  anterior  second  enlarged  por- 
tion. This  enlarged  portion  ends  anteriorly  in  the  uncus,  which  is 
the  anterior  extremity  of  the  limbic  lobe,  marked  oft3  by  a  narrow 
fissure,  the  dentate  fissure,  from  the  rest  of  the  limbic  lobe. 

The  Precuneus  —  Between  the  posterior  upturned  end  of  the 

226 


THE  NERVOUS  SYSTEM 

calloso- marginal  fissure  and  another  fissure,  the  subparietal  fissure, 
which  continues  the  general  curve  of  the  calloso-marginal  fissure 
around  corpus  callosum,  is  the  portion  of  the  parietal  lobe  which 
appears  on  the  internal  surface  of  the  hemispheres.  This  portion  of 
the  parietal  lobe  is  called  the  precuneus.  That  portion  of  the 
parietal  lobe  appearing  on  the  internal  surface  of  the  hemispheres 
in  front  of  the  precuneus  is  called  the  lobulus  quadratus. 

The  Occipital  Parietal  Fissure  —  Behind  the  precuneus  is  the 
occipital  parietal  fissure,  which  separates  the  precuneus  from  the 
occipital  lobe. 

Calcarine  Fissure  —  The  occipital  lobe  is  divided  into  two  parts 
by  a  deep  fissure,  curving  downwards  and  backwards  from  the  mid- 
dle of  the  occipital  parietal  fissure  toward  the  posterior  pole  of  the 
brain.  This  fissure  is  called  the  calcarine  fissure. 

Above  the  calcarine  fissure  the  occipital  lobe  is  called  the  cuneus 
and  below  but  more  anteriorly  the  lobulus  lingualis. 

Collateral  Fissure  —  Beneath  the  lobulus  lingualis,  separating 
it  and,  more  anteriorly,  the  limbic  lobe  from  the  portion  of  the  tem- 
poral lobe  which  appears  on  the  internal  surface  of  the  hemisphere, 
is  the  collateral  fissure.  It  runs  horizontally  between  the  lobes 
which  it  separates.  The  dentate  fissure,  a  small  fissure  in  the  limbic 
lobe  above  and  parallel  to  the  collateral  fissure  produces  the  promi- 
nence of  the  hippocampus  major  upon  the  inner  wall  of  the  inferior 
cornu  of  the  lateral  ventricle.  The  calcarine  fissure  produces  the 
eminence  of  the  calcar  avis  on  the  inner  wall  of  the  posterior  cornu 
of  the  lateral  ventricle. 

The  Fornix  —  The  bundle  of  fibers  forming  the  f ornix  terminate 
posteriorly  in  hippocampus  major  and  eminentia  collateralis  — 
structures  to  be  mentioned  later,  and  appearing  in  the  floor  of  the 
descending  horn  of  the  lateral  ventricle  and  posterior  to  the  optic 
thalamus.  They  are  continued  through  the  synapses  of  the  corpora 
albicans  as  another  bundle,  the  bundle  of  Vicq  d  'Azyr,  which  curves 
directly  out  of  the  corpora  albicans  into  the  optic  thalami. 
(Fig.  96.) 

The  Character  of  the  Cortex  —  The  external  walls  bounding  the 
lateral  ventricles  as  a  whole,  i.e.,  the  later  transformation  of  the 
precerebral  vesicle,  become  very  much  thickened  in  all  aspects 
except  the  internal,  in  other  words,  above,  externally,  below,  in 

228 


THE  NERVOUS  SYSTEM 


230 


THE  NERVOUS  SYSTEM 

front  and  behind.  This  thickening,  thrown  into  folds  on  its  outer 
surface,  constitutes  the  cortex  and  white  matter  of  the  cerebrum. 

The  Fifth  Ventricle  —  In  front  of  the  foramen  of  Monro  the 
brain  cortex  becomes  coapted  and  united  in  such  manner  that  it 
incloses  a  hollow  cavity,  which  therefore  at  no  time  was  a  part  of 
the  system  of  original  cerebral  vesicles. 

This  cavity  is  called  the  fifth  ventricle  of  the  brain.  Its  walls, 
which  are  formed  of  thin  layers  of  gray  matter,  are  called  the  sep- 
tum lucidum. 

The  Relation  of  the  Optic  Thalami  to  the  Lateral  Ventricles  — 
The  primarily  posterior  and  later  internal  walls  of  the  lateral  ven- 
tricle inclose  the  optic  thalamus  of  the  corresponding  side  by 
curving  around  the  latter.  The  anterior  horn  curves  around  the 
anterior  rounded  end  of  the  optic  thalamus  and  the  inferior  horn 
curves  around  the  posterior  extremity  of  the  optic  thalamus  and 
so  completely  that  at  the  origin  of  this  horn  its  floor  is  formed  by 
the  optic  thalamus,  while  at  its  extremity  its  roof  is  formed  of  the 
optic  thalamus. 

The  posterior  horn  curves  around  in  an  external  direction  the 
posterior  extremity  of  the  optic  thalamus  largely  in  the  same  hori- 
zontal plane  as  that  of  the  body  of  the  lateral  ventricle. 

THE   INTERNAL    STRUCTURE   OF    THE    BRAIN 

There  are  two  important  differences  between  the  internal  struc- 
ture of  the  cord  and  that  of  the  medulla,  which  represent  changes 
of  development  undergone  by  the  medulla  from  the  manner  in 
which  the  cord  develops. 

The  first  of  these  is  the  displacement  of  the  central  canal  pos- 
teriorly until  it  no  longer  forms  a  canal  but  a  median  groove  upon 
the  floor  of  the  fourth  ventricle.  The  second  change  is  the  cutting 
up  of  the  gray  matter  of  the  anterior  horns  by  fibers  of  the  pyram- 
idal tracts  crossing  the  middle  line  arid  decussating  with  each  other 
until,  practically  entirely  crossed,  they  occupy  a  situation  on  each 
side  of  the  middle  line  producing  two  rounded  eminences  upon  the 
anterior  surface  of  the  lower  half  of  the  medulla,  immediately  be- 
neath the  pons. 

As  the  central  canal  of  the  spinal  column  opens  up  into  the 
medulla,  the  gray  matter  of  the  posterior  horns  becomes  displaced 

232 


THE  NERVOUS  SYSTEM 

laterally  and  the  gray  matter,  now  in  part  interspersed  between  the 
fibers  of  the  crossing  pyramidal  tracts,  also  becomes  displaced  to  a 
position  on  each  side  of  the  middle  line  in  the  floor  of  the  fourth 
ventricle.  Hence  it  is  that  the  sensory  nuclei  of  the  cranial  nerves 
always  occupy  a  more  lateral  position  than  the  motor  nuclei.  The 
cranial  nerves  may  be  divided  into  motor  nerves  and  sensory  nerves. 
A  number  of  them  have  both  sensory  and  motor  roots. 


Fig.  97. — Diagrams  illustrating  the  origin  and  relations  of  the  root-fibres  of 

the  cerebral  nerves.     (Quain.) 

A,  efferent  fibres  only;  lateral  view.  B  shows  on  the  left  the  motor  nuclei 
and  efferent  fibres,  except  those  of  the  fourth  nerve,  and  on  the  right  side 
the  afferent  fibres;  surface  view. 

The  Nuclei  and  Superficial  Origin  of  the  Motor  Cranial  Nerves 

(Figs.  97-100)  — From  below  upwards  the  motor  nerves  are  the 
twelfth,  the  seventh,  the  motor  portion  of  the  sixth,  the  fifth,  the 
fourth  and  the  third.  The  nuclei  of  the  twelfth  and  sixth  nerves  lie 
in  the  gray  matter  of  the  floor  of  the  fourth  ventricle,  close  to  the 
middle  line,  one  below  and  the  other  above  the  striae  acusticae  in  just 
the  position  which  the  displaced  gray  matter,  corresponding  to  the 
anterior  horns  of  the  spinal  column,  should  occupy  as  the  conse- 

234 


THE  NERVOUS  SYSTEM 

quence  of  the  opening  out  process  of  the  central  canal  of  the  spinal 
column.  The  nerve  fibers  arising  from  the  cells  of  these  become 
collected  in  bundles  which  pass  outwards  and  forwards  to  emerge  in 
a  series  of  roots  in  the  groove  between  the  pyramids  and  the  olivary 
body. 

In  the  same  manner  the  sixth  nerve  emerges  from  the  medulla  at 
the  lower  border  of  the  pons  at  the  upper  end  of  the  same  groove. 

In  direct  line  with  these  nuclei,  close  to  the  iter  of  Sylvius,  is  the 

Nucleus   of   tractus   solitarius. 

Nucleus   of   ala  cinerea.          i  Medial  nucleus  and  descending  root  of 

vestihular  nerve. 

Nucleus  of  fasciculus  cuneatus. 

s  Nucleus  ambiguus. 
-.    Restiforra  body. 


Root  filum  of  vagus  cerebello- 
olivary  fibres. 


Ventral  external  arcuate  fibres 


Fig.  98. — Diagram  showing  tEe  composition  of  the   jerebellar  portions  of  the 
internal  and  external  arcuate  fibres.     (Morris.) 


column  of  nerve  cells  forming  the  nuclei  of  the  fourth  and  third 
nerves.  The  fibers  of  the  fourth  nerve  become  collected  into  a 
bundle  which  passes  backwards  along  the  outer  side  of  the  nucleus 
until  they  reach  the  upper  limits  of  the  medulla  where  they  decus- 
sate and,  after  the  crossing,  emerge  on  each  side  from  the  groove 
at  the  lower  margin  of  the  inferior  corpora  quadrigemina,  between 
this  latter  and  the  superior  peduncle  of  the  cerebellum.  The  other 
cranial  nerves  possess  motor  and  sensory  roots,  but  the  nuclei  of 
the  motor  roots  always  lie  internal  to  those  of  the  sensory  roots. 
Thus  it  is  that  nucleus  of  the  vagus  or  tenth  nerve,  which  is  partly 
motor  and  partly  sensory,  lies  under  the  ala  cinerea  external  to  the 
position  of  the  origin  of  the  twelfth  nerve.  The  motor  portion  of 
the  tenth  arises  from  a  separate  nucleus,  the  nucleus  anibiguus, 

236 


THE  NERVOUS  SYSTEM 


:/  Nucleus  of  olfactory  nerve. 


Nucleus    of    oculomotor  nerve. 
Nucleus  of  trochlear  nerve 
Nucleus    of    mesencephalic 

root  of  masticator. 
Chief  motor  nucleus  of 
masticator. 


Nucleus   of   facial. 

Nucleus  of  abducens.- 

Nucleus    ambiguus     (vagus, 

and      glossopharyngeus). 

Nucleus  of  hypoglossus 


Nucleus  of  spinal   accessory  nerve.  .  - 


\  Pulvinar  of  thalamus. 

Lateral  geniculate  body. 
Nucleus  of  superior  colliculus, 

or  corpus  quadrigeminum. 
-\"  Sensory  nucleus  of  trigeminus. 
Nucleus  of  vestlbular  nerve. 

Ventral  nucleus  of 
cochlear    nerve. 

-  - '  Dorsal  nucleus  of  cochlear  nerve. 


Nucleus     alse     cinereae     (vagus     and 
glossopharyiigeus) . 

Solitary  tract  (vagus  and 
?J/"~~     glossopharyngeus). 

—-Nucleus  of  spinal  tract  of  trigeminus. 


Fig.   99. — Scheme   showing  the  relative  size  and  position  of  the  nuclei  of 

origin    of    the    motor    and    the    nuclei    of    termination 

of  the  sensory  cranial  nerves.     (Morris.) 


238 


THE  NERVOUS  SYSTEM 


Insula. 


Anterior  perforated 
substance. 


Mammillary   bodies. 


Cerebral  peduncle 


Semilunar  (Gasserian)  • 

ganglion. 


Oblique  fasciculus  of 
pons. 


Olfactory  tract. 

i  Hypophysis. 


— — —  Optic  nerve. 
Optic  tract. 


Tuber  cinereum. 

Oculomotor 

nerve  (III),     j 

— _  Lateral    genicu-    j 

late  body. 
Trochlear      nerve 
-       (IV). 

Masticator  .nerve 
(motor  root  oft 
trigeminus).     ) 
"""^Trigeminus   (V).J 

Abducens    (VI). 


Hypoglossal  nerve  (XII). 


Brachium  of 

Pons. 
Facial    nerve 

(VII). 
.  Glosso-palatine 

(intermediate   pai 
of  facial). 
Cochlear      and      v« 

tibular     n  e  r  v  e  sjj 
\  (acoustic  or  VIII). j 

Glossopharyngeal 
nerve   (IX). 

Vagus  nerve   (X). 


Accessory  nerve   (XI) 
(spinal  accessory). 


v Cervical  I. 


Cervical  II. 


Pyramid. 
Decussation  of  pyramids. 

Fig.  100. — Semi-diagrammatic  representation  of  the  ventral  aspect  of  the  rhombencephah 
and  adjacent  portions  of  the  cerebrum.    (Morris.) 


240 


THE  NERVOUS  SYSTEM 

which  lies  internal  to  the  sagittal  plane  of  the  main  nucleus.  The 
fibers  of  the  third  nerve  pass  directly  outward  and  ventral-wards, 
traversing  the  substance  of  the  mid-brain  to  emerge  close  to  the 
middle  line  on  the  ventral  surface  of  the  mid-brain  between  the 
diverging  crura  cerebri. 

The  seventh  nerve  arises  from  a  nucleus  external  and  ventral  to 
and  slightly  below  the  nucleus  of  origin  of  the  sixth  nerve,  deeply 
placed  beneath  the  floor  of  the  upper  half  of  the  fourth  ventricle. 
Its  nucleus  would  seem  at  first  sight  to  be  too  far  laterally  placed 
for  a  motor  nucleus,  but  its  deeper  position  in  the  recticular  forma- 
tion explains  this  apparent  irregularity,  for  it  must  be  remembered 
that  all  the  motor  nuclei  of  the  cranial  nerves  occupied  originally 
the  position  of  the  laterally  placed  anterior  horns  and  in  the  open- 
ing out  process  of  the  central  canal  of  the  spinal  column  they  are 
at  first  displaced  from  a  lateral  position  to  an  internal  one.  The 
seventh  nerve  possesses  also  a  sensory  root,  which  maintains  its 
integrity  as  a  separate  bundle  of  fibers  at  the  superficial  origin  of 
the  nerve.  Its  incoming  fibers,  like  other  sensory  nerves,  divide  into 
ascending  and  descending  fibers  which  terminate  around  cells  con- 
tinuing the  column  of  the  ninth  nerve  further  upwards.  The  fibers 
of  the  main  portion  of  the  seventh  nerve  are  motor  and  become  col- 
lected into  a  bundle  which  forms  a  peculiar  curve,  at  first  down- 
wards and  inwards  and  backwards,  then  directly  upwards  and  then 
downwards  and  outwards  and  forwards  in  a  manner  to  completely 
encircle  the  nucleus  of  the  sixth  nerve.  It  finally  emerges  in  the 
groove  between  the  pons  and  the  medulla  just  anterior  to  the  posi- 
tion of  the  superficial  origin  of  the  eighth  nerve. 

The  Sensory  Nuclei  —  It  must  of  course  be  remembered  that 
the  nuclei  of  the  sensory  nerves  are  not  to  be  viewed  as  nuclei  of 
origin,  as  is  the  case  with  the  nuclei  of  the  motor  nerves. 

The*  nuclei  of  the  sensory  nerves  are  collections  of  nerve  cells 
around  which  the  termination  of  the  sensory  fibers  arborize. 
Though  more  deeply  placed,  the  nucleus  of  the  ninth  nerve  simply 
continues  upwards,  the  column  of  cells  of  origin  of  the  vagus 
underlying,  in  the  floor  of  the  fourth  ventricle,  an  area  beginning 
in  the  gray  matter  of  the  ala  cinereum  and  extending  upwards 
external  to  the  trigonum  hypoglossi  to  nearly  the  level  of  the  striae 
acusticae. 

The  fifth  nerve  possesses  a  separate  motor  and  sensory  portion. 

242 


THE  NERVOUS  SYSTEM 

The  position  of  its  nuclei  follows  the  general  rule  of  the  other 
cranial  nerves.  The  motor  nucleus  is  situated  at  a  little  depth 
below  the  uppermost  portions  of  the  pontine  part  of  the  medulla, 
at  its  extreme  lateral  portion. 

This,  however,  is  not  a  very  great  distance  from  the  middle 
line,  inasmuch  as  the  fourth  ventricle  is  quite  narrow  at  this  level. 
The  column  of  cells  of  the  main  portion  of  the  motor  nucleus  is 
continued  brainwards,  forming  a  streak  of  gray  matter  external 
and  ventral  to  the  nuclei  of  the  fourth  and  third  nerve  from 
which  fibers  run  downwards  forming  one  bundle  with  the  fibers  of 
the  main  motor  nucleus.  The  collected  fibers  of  the  motor  portion 
of  the  fifth  nerve  emerge  from  the  lateral  surface  of  the  pons. 

The  incoming  fibers  of  the  sensory  portion  of  the  fifth  nerve 
enter  the  pons  immediately  below  the  motor  root.  They  traverse 
the  substance  of  the  pons  and  divide  into  ascending  and  descend- 
ing bundles.  The  ascending  bundles  pursue  a  shorter  course  and 
terminate  around  cells  forming  a  nucleus  which  lies  near  the 
lateral  margin  of  the  pons,  lateral  to  the  motor  nucleus,  though 
not  extending  iriuch  above  the  level  of  the  upper  limits  of  the 
fourth  ventricle. 

The  descending  fibers  run  downwards  for  a  very  long  distance, 
no  less  than  as  far  as  the  level  of  the  second  cervical  nerve.  This 
descending  root  occupies  a  position  at  first  in  the  lateral  boun- 
daries of  the  pons  in  the  substance  of  the  transversely  running 
fibers  of  this  structure.  In  lower  levels  it  lies  close  to  the  super- 
ficial lateral  surface  of  the  medulla  internal  and  posterior  to  the 
corpus  restiforme  and  crossed  laterally  by  the  fibers  of  the  eighth 
nerve.  Still  lower  it  forms  a  cap  at  the  tubercle  of  Rolando  and 
the  substantia  gelatinosa  of  Rolando,  and  may  be  traced  as  far 
down  as  the  second  cervical  vertebrae.  Most  of  its  fibers  terminate 
in  the  chief  sensory  nucleus,  situated  dorsally  to  it  in  the  upper 
level  of  the  pons,  lateral  and  ventral  to  the  position  of  the  motor 
nucleus,  coming  very  close  to  the  lateral  surface  in  an  area  indi- 
cated by  the  angle  formed  by  the  superior  and  middle  peduncles 
of  the  cerebellum. 

The  Eighth  Cranial  Nerve  —  The  only  remaining  cranial  nerve, 
exclusive  of  the  optic  and  olfactory  tracts,  which  are  not  peripheral 
nerves  at  all  but  bundles  of  nerve  fibers  comparable  to  intra- 
cerebral  tracts,  is  the  eighth  cranial  nerve.  It  is  proper  to  consider 

244 


THE  NERVOUS  SYSTEM 

this  nerve  in  a  class  by  itself  or  with  the  optic  and  olfactory  nerves 
as  it  is  so  specially  proprioceptive  in  its  character  that  it  stands 
quite  apart  from  the  other  cranial  nerves. 

The  eighth  cranial  nerve  is  composed  of  two  portions,  entirely 
different  in  function.  Both  arise  in  the  sensory  cells  of  the  in- 
ternal ear.  One  hundle  of  fibers  constitutes  the  auditory  division 
of  the  nerve  and  the  other  the  vestibular  division.  Both  divisions, 


n.XE 


Fig.  101. — Transverse  section  at  the  upper  part  of  the  medulla   oblongata. 

(Quain.) 

Py,  pyramid;  o,  olivary  nucleus;  d.  V.,  descending  root  of  the  fifth  nerve; 
VIII,  root  of  the  acoustic  nerve,  formed  of  two  parts,  a  (cochlear)  and  b 
(vestibular),  which  enclose  the  restiform  body,  c. r.;  n.VIIIp,  dorsal  nucleus 
of  the  vestibular  nerve;  n.  VIII  ac,  ventral  acoustic  nucleus;  g,  ganglion-cells, 
of  the  acoustic  tubercle  (lateral  acoustic  nucleus);  n.f.t.,  nucleus  of  the 
funiculus  teres;  n.XII,  nucleus  of  the  hypoglossal;  r,  raphe. 

however,  enter  the  medulla  just  beneath  the  pons  Varolii  as  one 
nerve,  parted  as  they  enter  the  medulla  into  a  dorsal  and  ventral 
division  which  inclose  between  them  the  restiform  body.  (Fig. 
101.)  However,  it  is  only  the  auditory  fibers  which  divide  to  in- 
close the  restiform  body.  All  of  the  vestibular  fibers  pass  meso- 
ventral  to  this  structure.  After  it  has  entered  the  medulla  the  ves- 
tibular division  divides,  like  other  sensory  nerves,  into  an  ascending 
and  descending  portion.  Both  divisions  pass  to  the  cells  underlying 

246 


THE  NERVOUS  SYSTEM 

the  area  in  the  floor  of  the  fourth  ventricle,  termed  the  trigonum 
acusticum. 

The  ascending  division  passes  to  the  upper  portion  and  the 
descending  division  to  the  lower  portion.  From  these  fibers  col- 
laterals join  two  other  important  nuclei  —  the  nuclei  of  Bechterew 
and  Deiters,  placed  internal  and  ventral  to  the  restiform  body. 
Many  fibers  of  the  vestibular  nerve  end  directly  around  the  cells 
of  these  nuclei.  The  fibers  of  the  auditory  division  of  the  eighth 


tub.ac. 


FIBRES  TO  NUCL.LEMNISCI 
&CORPORA  QUADRIGEMINA 


NERVE-ENDINGS 

IN  ORGAN  OF  CORTI 

Fig.  102. — Plan  of  the  course  of  connections  of  the  fibres  forming  the  cochlear 

root  of  the  auditory  nerve.     (Quain.) 

r.,  restiform  body;  V,  descending  root  of  the  fifth  nerve;  tub.ac.,  tuber- 
culum  acusticum ;  n.  ace.,  accessory  nucleus ;  s.  o.,  superior  olive ;  n.  tr.,  nucleus 
of  trapezium ;  n.  VI,  nucleus  of  sixth  nerve ;  VI,  issuing  root-fibre  of  sixth 
nerve. 

nerve  divide  to  inclose  the  restiform  body.  The  dorso-lateral 
fibers  end  around  cells  forming  a  prominence,  the  tuberculum  acus- 
ticum, on  the  posterior  surface  of  the  restiform  body,  just  above 
the  trigonum  acusticum  and  in  many  cells  interspersed  among  the 
fibers  of  the  dorsal  division  itself.  The  striae  acusticae  themselves 
are  composed  of  fibers  originating  as  the  axis  cylinders  of  these 
nerve  cells.  They  pass  internally,  crossing  the  middle  line  and, 
therefore,  the  fibers  of  the  opposite  side.  As  soon  as  they  have 
crossed  to  the  opposite  side  they  dip  down  close  to  the  middle 
line  to  enter  the  deep  portions  of  the  medulla  to  be  continued  to 

248 


THE  NERVOUS  SYSTEM 

the  inferior  corpora  quadrigemina  in  a  manner  to  be  subsequently 
described.     (Fig.  102.) 

The  fibers  of  the  auditory  nerve,  which  pass  meso-ventrally  to 
the  corpus  restiforme,  end  around  cells  to  the  inner  and  ventral 
side  of  this  body,  for  the  most  part  placed  between  the  auditory 
and  vestibular  divisions  of  the  eighth  nerve.  Higher  up  these 
cells  become  continuous  with  the  nuclei  of  the  meso-ventral  divi- 
sion. From  these  nuclei  fibers  also  arise  which  cross  the  middle 


TO  VERMIS 


TO  HEMISPHERE 


FIBRES    OF 

VESTIBULAR 

ROOT 


NERVE 
ENDINGS 
IN  MACULXE 
&AMPULL/E 


Fig.  103. — Plan  of  the  course  and  connections  of  the  fibres  forming  the  ves- 
tibular root  of  the  auditory  nerve.     (Quain.) 

r.,  restiform  body;  V,  descending  root  of  fifth  nerve;  p.,  principal  nucleus 
of  vestibular  root ;  d,  fibres  of  descending  vestibular  root ;  n.  d.,  a  cell  of  the 
descending  vestibular  nucleus;  D,  nucleus  of  Deiters;  B,  nucleus  of  Bech- 
terew;  n.  t.,  nucleus  tecti  (fastigii)  of  the  cerebellum;  plb.,  posterior  (dorsal) 
longitudinal  bundle. 

line,  deeply  decussating  in  the  medulla  in  a  manner  to  be  later 
described  and  ultimately  reach  the  inferior  corpus  quadrigeminum 
of  the  opposite  side.  (Figs.  102  and  103.) 

We  have  now  considered  the  change  produced  in  the  medulla 
by  the  opening  out  pf  the  central  spinal  canal,  and  the  effect  which 
this  change  has  produced  upon  the  location  of  the  nuclei  of  the 
cranial  nerves. 

The  Nuclei  Cuneatus  and  Gracilis  —  It  now  remains  to  con- 

250 


THE  NERVOUS  SYSTEM 


sider  the  new  nuclei  appearing  throughout  the  medulla  and  mid- 
brain  and  the  further  course  through  the  medulla  and  mid-brain 
of  the  axis  cylinders  of  these  nuclei,  of  the  nuclei  of  the  afferent 
cranial  nerves  and  of  other  great  sensory  tracts.  The  first  impor- 
tant new  masses  of  gray  matter  met  with  are  the  nuclei  cuneatus 
and  gracilis,  at  the  level  of  the  lower  half  of  the  medulla  oblongata 


Funiculus  gracilis 
Dorsal  median  fissure. 


Funiculus  cuneatus. 
Nucleus  gracilis. 


Descending  root  of  Vth. 
Bundle  from  funiculus 
cuneatus. 
Substantia  Rolandl. 


Bundle  of  Flechsig. 
Pyramid-tract  bundles. 


Decussation  of  pyramids. 

Caput  cornu  ventralis. 

Ventral  median  fissure. 

Pyramid. 

Fig.   104. — Section   across  the  lower  part  of  the  medulla   oblongata  in  the 

middle  of  the  decussation  of  the  pyramids.     Magnified 

about  six  diameters.     (Quain.) 

situated  on  its  dorso-external  aspect,  external  to  the  fourth 
ventricle.  They  receive  around  their  nerve  cells  the  terminal 
arborizations  of  the  fibers  of  the  posterior  columns  of  Goll  and 
Burdach  respectively.  (Figs.  104  to  107.) 

Tubercle  of  Rolando  —  Another  nucleus  of  gray  matter  ex- 
ternal and  ventral  to  the  nucleus  cuneatus  is  the  tubercle  of 
Rolando.  This  is  not  a  new  mass  of  gray  matter  but  it  will 
make  the  description  clearer  to  mention  it  at  this  place.  The 
tubercle  of  Rolando  is  merely  the  enlarged  upper  extremity  of  the 
gray  substance  of  the  substantia  gelatinosa  of  Rolando  around 

252 


THE  NERVOUS  SYSTEM 


the  posterior  horns.    Around  its  cells  doubtlessly  terminate  many 
fibers  of  the  descending  root  of  the  fifth  nerve. 

Corpus  Restiforme  (Figs.  109-111)— The  tubercle  of  Ro- 
lando appears  to  be  overlapped  at  its  upper  extremity  by  bundles 
of  fibers  (two  bundles  in  particular)  which  join  to  form  the  begin- 
ning of  the  inferior  peduncle  of  the  cerebellum  and  constitute  the 
corpus  restiforme.  This  body,  therefore,  is  in  the  lateral  aspect 


Ftmiculus  gracilis. 
Funiculus  cuneatus. 


Funiculus  Rolandi. 
Substantia  Rolandi. 


Bundle  of  Flechsig. 
Lateral  nucleus. 


Caput  cornu  ventralis. 


Dorsal  median  fissure. 

Nucleus  gracilis. 
Nucleus  cuneatus. 


Central  canal. 


Decussation   of 
pyramids. 


Ventral  median 
fissure. 


Pyramid 


Fig.  105. — Section  across  the  medulla  oblongata  at  the  level  of  the  upper- 
most part  of  the  decussation  of  the  pyramids.     (Quain.) 

of  the  medulla,  just  below  the  pons  Varolii  and  just  above  the 
tubercle  of  Rolando  and  the  termination  of  the  column  of  Burdach 
in  the  nucleus  cuneatus. 

Olivary  Nucleus  —  A  third  new  mass  of  gray  matter  appear- 
ing in  the  medulla  is  the  olivary  nucleus.  It  presents  a  wavy 
appearance  on  cross  section,  arranged  in  a  curved  manner,  concave 
internally,  and  produces  a  very  decided  prominence  between  the 
prominences  of  the  pyramids  and  the  tubercle  of  Rolando  imme- 
diately below  the  pons  Varolii. 

Superior  Olive  —  The  fourth  new  mass  of  gray  matter  is  the 

254 


THE  NERVOUS  SYSTEM 

superior  olivary  nucleus,  smaller  and  situated  above  the  main 
olivary  nucleus,  in  among  the  transversely  coursing  fibers  of  the 
pons  Varolii  itself. 

Formatio  Reticularis  —  The  transversely  running  fibers  of  the 
pons  Varolii  form  a  large  portion  of  the  pontine  portion  of  the 


Gracile  nucleus 


Fasciculus  cuneatus 


Cuneate  nucleus. 

Tractus  solitarius. 
Tractus      spinalis      of 

trigeminal    nerve. 
Nucleus       of       tractus 

spinalis    of    trigemi- 

nal  nerve. 
Internal   arcuate   fibre. 

Fila.  of  hypoglossal 
nerve. 


External  arcuate  fibres. 


Inferior    olivary 
nucleus. 


Medial  accessory   olivary 
nucleus. 

Pyramid. 


Central  canal. 
Hypoglossal  nucleu> 


Fasciculus 

longitudinal  is 

medialis. 
Hypoglossal  nerve. 


Raphe. 

Medial  lemniscus. 


External  arcuate 
fibres. 


Fig.   106. — Transverse  section  through  the  middle  of  the  olivary  region   of 

the   human   medulla   oblongata.      (Cunningham.) 

The  floor  of  the  fourth  ventricle  is  seen,  and  it  will  be  noticed  that  the 
restiform  body  on  each  side  has  now  taken  definite  shape. 


medulla.  They  are  composed  of  a  large  number  of  interlacing 
fibers  passing  between  the  two  hemispheres  of  the  cerebellum. 
A  portion  of  these  fibers  are  the  pyramidal  tracts  on  their  way 
from  the  brain  to  the  spinal  cord.  In  other  words,  the  fibers 
of  the  pyramidal  tracts  plunge  deeply  into  the  pons  Varolii  and 
become  covered  and  broken  up  by  the  transverse  fibers  of  this 
structure  before  they  become  united  again  to  form  the  pyramids 
just  above  their  decussatioi*  Nevertheless,  many  of  the  pyram- 

256 


THE  NERVOUS  SYSTEM 

idal  fibers  remain  collected  in  the  pons  in  a  fairly  well-defined 
bundle  near  the  anterior  surface  of  the  pons.  More  dorsally 
other  transverse  fibers,  which  at  higher  levels  become  longi- 
tudinal, spring  from  the  nuclei  cuneatus  and  gracilis.  Still  other 
transverse  fibers  cross  the  middle  line  from  each  olivary  nucleus 
and  from  each  Deiters'  nucleus.  All  these  fibers,  with  others  origi- 


Pasciculus  cuneatus. 
Vestibular  nucleus. 
Restiform  body. 
Fasciculus  solltarus. 


Bundle  of  Flechsig.      • 

Descending  root  of  Vth.  ~^E9 

Substantia  Rolandi.  ~g||| 

Part  of  descending  • 

root  of  Vth. 
Internal  arcuate  fibres.   ... 

Fibres  of  Xth 

Bundle  of  Gowera.  r 

Raphe.-— 
Thalamo-olivary  tract.—-— 

Accessory  olivary 
nucleus. 
Olivary  nucleus. 

Fibres  of  Xllth  nerve.  - 
External  arcuate  fibres. 


Pyramid. 
Arcuate  nucleus. 


Dorsal 

longitudinal 
bundle. 

Ventral 

longitudinal 
bundle. 


Fig.  107. — Section  across  medulla  oblongata  a  little  above  the  level  of  the 
point  of  the  calamus  scriptorius.    Magnified  about  six  diameters.    (Quain.) 


nating  from  scattered  cells  among  the  fibers  themselves,  form  a 
confused  network  dorsal  to  the  main  mass  of  fibers  of  the  pons 
Varolii  and  constitute  what  is  known  as  the  'formatio  reticularis. 
The  Cerebellum  —  The  gray  matter  of  the  cerebellum,  with 
its  contained  nuclei,  must  also  be  considered  as  additional  masses 
of  gray  matter  added  to  the  primitive  segmented  cerebrospinal 
axis  of  the  invertebrates.  As  explained,  it  is  connected  by  two 
superior,  two  middle  and  two  inferior  peduncles,  with  respectively 
the  mid-brain,  the  medulla  and  the  fourth  ventricle.  The  cerebel- 

258 


THE  NERVOUS  SYSTEM 


260 


THE  NERVOUS  SYSTEM 


262 


THE  NERVOUS  SYSTEM 


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264 


THE  NERVOUS  SYSTEM 

lum  itself  is  composed  of  two  lateral  hemispheres  and  a  central 
lobe  which  latter  appears  as  rounded  eminences  on  the  superior 
and  inferior  surface  of  the  cerebellum  between  the  lateral  hemi- 
spheres. These  eminences  are  termed  the  superior  and  inferior 


Fig.  111. — Transverse  section  of  pons  through  the  origin  of  the  auditory  nerve. 

From  a  photograph.  Magnified  about  four  diameters.  (Quain.) 
v.  IV,  fourth  ventricle;  c.,  white  matter  of  cerebellar  hemisphere;  c.  d., 
corpus  dentatum  cerebelli ;  fl.,  flocculus ;  c.  r.,  corpus  restiforme ;  R,  Roller's 
"ascending"  auditory  bundle  (really  formed  of  descending  fibres  of  vestibular 
nerve);  D,  Deiters'  nucleus;  VIII,  root  of  auditory  nerve;  VIII d.,  principal 
nucleus  of  vestibular  division;  VIII  v.,  ventral  nucleus  of  cochlear  nerve; 
n.  tr.,  small-celled  nucleus  traversed  by  fibres  of  the  trapezium ;  tr.,  trapezium ; 
/.,  mam  fillet;  p.l.b.,  posterior  or  dorsal  longitudinal  bundle;  j.r.,  formatio 
reticularis ;  n,  n' ,  n" ,  nuclei  in  formatio  reticularis ;  V.  a.,  so-called  ascending 
root  of  fifth  (really  descending);  s.  g.,  substantia  gelatinosa;  s.  o.,  upper 
olivary  nucleus ;  VII,  issuing  root  of  facial ;  n.  VII,  nucleus  of  facial ;  VI, 
root-bundles  of  abducens;  py.,  pyramid-bundles;  n.p.>  nuclei  pontis. 

vermis.  (Figs.  120-122.)  The  entire  surface  of  all  lobes  is  com- 
posed of  gray  matter,  thrown  into  folds  for  the  purpose  of  increas- 
ing its  surface. 

Its  Nuclei  —  In  the  center  of  each  lateral  hemisphere  is  placed 

266 


THE  NERVOUS  SYSTEM 


Tractus 
spinalis   of 
trigeminal 
nerve. 

Its    nucleus. 

Facial  nerve. 

Facial  nucleus. 

Superior    olive. 

Fasciculus     v, ,\ 
thalamo- 
olivaris. 

I.emniscus  . 
medialis. 


Brachlum  pontis. 


Nucleus    of 

tractus. 
Spinalis    of 

trigeminal 

nerve. 
Vestibular 

nerve. 
Tractus 

spinalis    of 

trigeminal 

nerve. 
Facial  nucleus. 

Facial    nerve. 
Superior    olive. 


Corpus 
trapezoldeum. 


Deep  transverse 
fibres  of  pons. 


Pyramidal  bundles. 
Superficial  transverse  fibres  of  pons. 

Fig.  112. — Section  through  the  lower  part  of  the  human  pons  immediately  above  the  medulla 

oblongata.    (Cunningham.) 


268 


THE  NERVOUS  SYSTEM 


a 
I 


•a 


270 


THE  NERVOUS  SYSTEM 


Upper  and  fourth  ventricle 

Mesencephalic  root  of   the 
trigeminal  nerve. 

Medial  longitudinal 
bundle. 

Formatio   reticularis. 


Anterior  medullary  velum. 
Gray  matter  on  floor  of 
fourth  ventricle. 
Brachium 

conjunctivum. 
Lemniscus 
lateralis. 


Commencing 
decussation 
of  brachia 
conjunctiva. 

Lemniscus 
medians. 


Pyram- 
idal 
bundles. 


Fig.  114. — Section  through  the  superior  part  of  the  pons  of  the  orang,  above 
the  level   of  the   trigeminal   nuclei.     (Cunningham.) 


272 


THE  NERVOUS  SYSTEM 


Root  bundle  of  IVth 
Ace.  motor  root  of  Vth. 


Sup.  cerebell.  ped. 

Part  of  lateral  fillet. 
Dorsal  long,  bundle. 

Ventral  long,  bundle. 


Lateral  fillet. 
Decuss.  of  superior  peduncles 

Main  fillet 


Substantia  nigra. 
Central  nucleus. 


Crusta  or  pes  peduncul 


Breaking   up    of   crusta   Into   pyramid 
bundles. 


Fig.  115. — Transverse  section  through  the  uppermost  part  of  the  pons. 

(Quain.) 


274 


THE  NERVOUS  SYSTEM 


Fig.  116. — Transverse  section  across  the  mid-brain  through  the  posterior  cor- 
pora  quadrigemina.     Magnified  about  3%    diameters. 

From  a  photograph.     (Quain.) 

gr.,  dorsal  quadrigeminal  groove  (sulcus  longitudinalis) ;  c.  q.  p.,  corpus 
quadrigeminum  posterius;  str.L,  stratum  lemnisci;  c.  gr.,  central  gray  matter; 
n.  HI,  IV,  oculomotor  nucleus ;  d.  V,  descending  motor  root  of  fifth  nerve ; 
p.l.b.,  posterior  longitudinal  bundle;  j.r.t.,  formatio  reticularis  tegmenti; 
d.  d' ,  decussating  fibres  of  tegmentum  (fountain-like  decussations  of  Forel 
and  Meynert) ;  s.  c.  p.,  decussating  fibres  of  superior  cerebellar  peduncles; 
/,  main  fillet;  /',  lateral  fillet;  pp.,  crusta  pedunculi;  s. n.,  substantia  nigra; 
g.i.p.,  interpeduncular  ganglion;  sy.,  Sylvian  aqueduct. 


276 


THE  NERVOUS  SYSTEM 


Central  gray 
matter. 


Aqueduct. 


Inferior  colliculus 

Mesencephalic  root  of  trigeminal  nerve. 
Nucleus  of  trochlear  ner?e. 
Brachiuna  inferjm* 

Medial  longitudinal  bundle 

Medial  lemniscus. 


Brachlum 
conjunct!  vum 


Basis  pedunculi. 


Fig.  117. — Transverse  section  through  the  human  mesencephalon  at  the  level 
of  the  inferior  colliculus.     (Cunningham.) 


278 


THE  NERVOUS  SYSTEM 


280 


S 


THE  NERVOUS  SYSTEM 


Fig.    119. — Section    across    the    mid-brain,    through    the    anterior    corpora 

quadrigemina.  Magnified  about  3%  diameters.  (Quain.) 
Sy.,  aqueductus  Sylvii;  c. p.,  commissura  posterior;  gl.pi.,  corpus  pinealis; 
c.  q.  a.,  gray  matter  of  one  of  the  anterior  corpora  quadrigemina ;  c.  g.  m., 
corpus  geniculatum  mesiale;  e.g. I.,  corpus  geniculatum  laterale;  tr.opt., 
tractus  opticus;  pp.,  pes  pedunculi ;  p.l.b.,  posterior  longitudinal  bundle; 
fi.,  upper  fillet;  r. n.,  red  nucleus;  n.III,  nucleus  of  third  nerve;  ///,  issuing 
fibres  of  third  nerve;  1. p. p.,  locus  perforatus  posticus. 


282 


THE  NERVOUS  SYSTEM 

an  important  nucleus  of  gray  matter,  the  dentate  nucleus.  On 
cross  section  it  appears  as  a  wavy,  curved  line  concentric  with 
the  surface  of  the  hemispheres. 

The  central  lobe  possesses  three  other  nuclei  on  each  side  of 
the  middle  line.  One,  the  nucleus  fastigii,  is  nearest  the  middle 
line  and  immediately  above  the  roof  of  the  fourth  ventricle.  A 
third  nucleus  lies  dorsal  to  this.  It  is  named  the  nucleus  globosus. 


Sulcus  prepyramidalls 


Uvula. 

Tonsilla. 

Lobulus  biventralis. 


Sulcus  intragracills 
Sulcus  postgracills. 


Sulcus  horizontalis  magnus. 


Lobulus  pos- 

tero-supei  ior. 
Lobulus  semi- 

lunaris  inferior. 
Lobulus  gracilis 

posterior. 
Lobulus  gracilis 

anterior. 
Pyramids. 


Fig.  120. — View  of  cerebellum  from  below.    Natural  size.     (Quain.) 


Between  it  and  the  dorsal  border  of  the  dentate  nucleus  is  still 
another  nucleus,  the  nucleus  emboliformis.  (Fig.  123.) 

The  Destination  of  the  Superior  Peduncles  of  the  Cerebellum 
—  After  decussation  the  majority  of  the  fibers  of  the  superior 
peduncle  of  the  cerebellum  terminate  in  the  red  nucleus:  The 
upper  termination  of  these  fibers  really  forms  a  capsule  to  the  red 
nucleus. 

The  Red  Nucleus  —  The  red  nucleus  is  situated  at  the  top  of 

284 


THE  NERVOUS  SYSTEM 


286 


THE  NERVOUS  SYSTEM 


288 


THE  NERVOUS  SYSTEM 

the  mid-brain,  beneath  and  ventral  to  the  corpora  quadrigemina, 
and  dorsal  to  the  inner  portion  of  crura  of  that  side.  Lateral  to 
it  and  dorsal  to  the  external  portion  of  the  cms  is  another  collec- 
tion of  gray  cells  termed  the  substantia  nigra.  (Figs.  118  and  119.) 
Substantia  Nigra  —  The  substantia  nigra  is  found  in  sections 
below  the  level  at  which  the  red  nucleus  is  formed.  It  separates 
the  crusta  of  the  cerebrum  from  a  large  mass  of  transversely  and 
longitudinally  running  fibers,  known  as  the  tegmentum  and  con- 


Fig.  123. — Section  across  the  cerebellum  and  medulla  oblongata  showing  the 
position  of  the  nuclei  in  the  medullary  centre  of  the  cerebellum.  (Quain.) 
n.  d.,  nucleus  dentatus  cerebelli ;  s,  band  of  fibres  derived  from  restif orm 
body,   partly   covering   the   dentate   nucleus ;   s.  c.  p.,   commencement   of  su- 
perior cerebellar  peduncle;   com',  com",  commissural  fibres  crossing  in  the 
median  white  matter. 


sisting  largely  of  fibers  making  up  the  superior  peduncles  of  the 
cerebellum. 

The  Tegmentum  —  Like  the  f ormatio  reticularis  the  tegmentum 
consists  of  many  interlocking  fibers,  definite  bundles  of  which 
belong  to  the  superior  cerebellar  peduncles.  It  also  contains  many 

290 


THE  NERVOUS  SYSTEM 

scattered  nerve  cells  which  form  relay  stations  for  some  fibers 
coming  from  higher  and  lower  levels. 

The  New  Tracts  of  White  Fibers  —  The  important  nuclei  of 
the  brain  stem,  the  medulla  and  mid-brain,  and  cerebellum,  have 
now  been  mentioned.  It  remains  to  describe  the  tracts  of  white 
fibers  connecting  them  and  passing  through  them.  It  will  be  con- 
venient to  start  with  the  various  tracts  of  white  matter  found  in 
the  spinal  cord,  though  it  must  always  be  kept  in  mind  that  those 
tracts  which  carry  impulses  in  a  descending  direction  are  being 

Nucleus  of  fasciculus 

cuneatus.  Nucleus  of  Commissural  nucleus  of  ala  cinerea. 

t    fasciculus  gracilis.     •         %  Dorsal  external  arcuate  fibres. 


—  Restiform   body. 


Spinal  tract  of  .  _ 
trigeminus. 


Ventral  external  arcuate 
••     fibres. 


Fig.   124. — Diagram  showing  the   composition   of  the   cerebellar  portions  of 
the  internal  and  external  arcuate  fibres.     (Morris.) 

tracted  in  a  direction  opposite  to  that  in  which  they  grow  and 
functionate,  and  toward  the  origin  of  the  axis  cylinders  of  which 
they  are  composed. 

Deep  Arcuate  Fibers  —  We  may  start  first  with  the  posterior 
spinal  columns,  the  column  of  Burdach  and  Goll,  carrying  sensa- 
tions of  muscular  sense  —  muscular  tone  and  reflex  coordination, 
which  reach  consciousness.  These  may  be  traced  to  their  endings 
around  the  cells  in  the  nucleus  cuneatus  and  gracilis.  From  these 
nuclei  other  fibers  are  given  off  which  pass  inward  and  ventrally 
through  the  lower  half  of  the  medulla  to  decussate  in  the  middle 
line  with  similar  fibers  of  the  opposite  side.  These  fibers  are  called 
the  deep  arcuate  fibers.  They  turn  upward  after  decussation,  lying 
close  to  the  middle  line  and  dorsal  to  that  portion  of  the  fibers  of 
the  pons  Varolii  which  surrounds  the  pyramids  as  they  pass  up- 

292 


THE  NERVOUS  SYSTEM; 


wards.     They  form  a  well-marked  bundle  in  this  situation  called 

the  mesial  fillet.    (Figs.  124-125  and  105  to  119.) 

The  mesial  fillet  may  be  traced  upwards  through  the  mid-brain 

where  it  occupies  a  more  lateral  position.    Above  the  pons  Varolii 

it  leaves  the  middle  line 
beneath  the  superior  pe- 
duncle of  the  cerebellum, 
and  at  higher  levels  is  lat- 
eral to  the  decussation  of 
the  superior  peduncles. 
The  mesial  fillet  termi- 
nates in  the  superior  cor- 
pora quadrigemina,  in  the 
external  geniculate  bodies 
and  in  the  optic  thalami. 
Superficial  Arcuate  Fi- 
bers—  A  second  set  of 
fibers  are  given  off  from 
the  nuclei  cuneatus  and 
gracilis,  passing  externally 
and  ventrically  instead  of 
internally.  These  are  the 
superficial  arcuate  fibers 
which  pass  over  the  tu- 
bercle of  Rolando,  over 
the  upper  portion  of  the 
olivary  prominence,  over 
the  pyramids,  over  the  op- 
posite olive  and  tubercle  of 
Rolando,  to  join  the  cor- 
pus restiforme  of  the  oppo- 


Pig.    125. — Diagram    of    the    spino-cerebel- 
lar,  bulbo-tegmental,  cerebello-tegmental, 


ponto  -  tegmental,    and    ponto  -  cerebellar 
tracts.     (Quain.) 

site  side.    A  number  of  the 

axons,  particularly  those  springing  from  a  little  accessory  cuneate 
nucleus  on  the  lateral  surface  of  the  main  nucleus,  join  the  resti- 
form  body  of  the  same  side.  As  the  restiform  body  forms  the 
inferior  peduncle  of  the  cerebellum  the  ultimate  termination  of 
these  fibers  is  to  the  gray  matter  of  this  portion  of  the  cerebellum. 
They  run  directly  to  the  cortex  particularly  of  the  vermis. 
(Fig.  124.) 

294 


THE  NERVOUS  SYSTEM 

The  Termination  of  the  Direct  and  Crossed  Cerebellar  Tracts 
—  Two  more  tracts  in  the  spinal  cord  convey  sensations  of  mus- 
cular tone  and  muscular  coordination.  They  are  the  direct  cere- 
bellar tract  and  the  anterior  cerebellar  tract.  The  former  convey 
the  uncrossed  muscular  sensations  which  do  not  reach  conscious- 
ness. Their  axons  originate  in  the  cells  of  Clark's  column  and 
they  pass  directly  into  the  corpus  restiforme  and  then  to  the  cere- 
bellum. The  antero-lateral  cerebellar  tract  carries  crossed  mus- 
cular sensations  which  do  not  reach  consciousness.  The  fibers  of 
this  tract  travel  upward  through  the  formatio  reticularis  of  the 
medulla  oblongata,  representing  the  only  longitudinal  spinal  fibers 
in  the  upper  part  of  the  pons  after  the  removal  of  direct  cerebellar 
tracts  and  the  posterior  columns,  with  the  exception  of  the  pyram- 
idal tracts.  That  portion  of  this  tract,  the  posterior  portion,  which 
conveys  muscular  sensations,  leaves  the  medulla  by  bending  directly 
dorsally  to  join  the  superior  peduncles  of  the  cerebellum,  passing 
with  them  to  the  cerebellum.  (Figs.  124  and  125.) 

The  Spino-thalamic  Fibers  —  Other  bundles  of  the  antero- 
lateral  column,  the  spino-thalamic  fibers,  convey  sensations  of  pain, 
of  heat  and  cold,  and  of  touch  and  pressure.  These  fibers  form 
the  column  of  Gowers  internal  to  the  crossed  cerebellar  tract  and 
a  bundle  anterior  to  it.  The  two  sets  of  fibers  join  the  medial 
fillet  and  end  with  this  bundle  in  the  optic  thalamus.  As  new 
masses  of  gray  matter  than  those  represented  in  the  cord  develop 
within  the  medulla  and  mid-brain,  so  also  new  tracts  of  fibers  are 
found  in  these  portions  of  the  brain.  (Fig.  125.) 

The  Connections  of  the  Olive  —  The  inferior  olivary  nuclei 
are  directly  connected  by  some  fibers  with  the  corpus  restiforme 
and  hence  with  the  cerebellum  of  the  same  side.  Most  of  the 
fibers,  however,  which  are  associated  with  the  olivary  nuclei, 
pass  across  the  middle  line  through  the  opposite  olive  and  into  the 
opposite  corpus  restiforme.  These  fibers  are  axis  cylinders  of  the 
olivary  bodies  —  at  least,  ablation  of  one  cerebellar  hemisphere 
will  cause  atrophy  of  the  opposite  olive.  It  is  possible  that  some 
of  the  olivo-cerebellar  fibers  may  be  efferent  from  the  cerebellum, 
as  some  fibers  originating  in  the  olivary  nucleus  pass  directly 
down  into  the  cord,  and  after  being  joined  with  other  fibers  from 
the  optic  thalamus  help  to  form  the  thalamic  or  olivo-spinal  tract 
of  Helweg.  This  tract  then  is  a  descending  tract,  but  it  has  been 

296 


THE  NERVOUS  SYSTEM 

convenient  to  describe  it  with  the  other  connections  of  the  olivary 
nucleus.     (Fig.  98.) 

The  Various  Fibers  Constituting  the  Corpus  Restifonne,  or 
Inferior  Peduncle  of  the  Cerebellum  —  Before  describing  other 
descending  tracts,  one  other  important  connection  to  the  restiform 
body  remains  to  be  described.  Some  of  the  fibers  of  the  vestibular 
branch  of  the  eighth  nerve,  which  are  connected  by  collaterals 
with  the  nuclei  of  Bechterew  and  Deiters,  form  a  bundle  known 
as  the  internal  restiform  'body.  The  internal  restiform  body  also 
contains  fibers  from  the  nuclei  of  the  glossopharyngeal  nerve  and 
probably  fibers  running  directly  from  the  nuclei  of  Bechterew  and 
Deiters.  This  bundle  joins  with  the  arcuate  fibers  and  passes  into 
the  corpus  restiforme  and  inferior  peduncle.  The  following 
bundles  of  nerve  fibers,  therefore,  run  to  the  cerebellum. 

1.  Fibers  originating  in  Clark's  column  of  cells,  homolateral 
and  ascending  in  the  direct  cerebellar  tract. 

2.  Fibers  from  the  dorsal  nuclei  of  the  posterior  columns  of 
the  same  and  opposite  side. 

3.  Internal  and   superficial  arcuate  fibers  from  the  olivary 
bodies. 

4.  From  the  vestibular  nerve,  the  glossopharyngeal  nerve  and 
Deiters  nucleus.    All  these  fibers  run  directly  to  the  cortex  of  the 
cerebellum,  particularly  the  cortex  of  the  vermis. 

The  Middle  Peduncle  of  the  Cerebellum,  or  the  Pons  Varolii 
—  The  cortex  of  cerebellar  hemispheres  receives  most  of  its  af- 
ferent fibers  from  the  middle  peduncle.  The  pons  Varolii  is  largely 
composed  of  fibers  which  are  only  commissural  and  run  from  one 
cerebral  hemisphere  to  the  other.  A  large  number  of  fibers  enter- 
ing the  middle  peduncle  are  axis  cylinders  of  cells  in  the  formatio 
reticularis;  others  are  efferent  and  end  around  cells  in  the  formatio 
reticularis.  Many  fibers  of  the  crura  cerebri  pass  between  the 
frontal  and  temporal  lobes,  and  the  formatio  reticularis  of  the 
opposite  side.  It  is  therefore  in  the  formatio  reticularis  that  one 
connection  between  the  cerebral  cortex  and  the  cerebellum  is 
effected. 

The  Afferent  Tracts  to  the  Cerebellum  in  the  Superior  Pe- 
duncle —  Two  other  afferent  tracts  enter  the  cerebellum.  They 
have  been  previously  mentioned.  One  ascends  from  the  cord  in 
the  lateral  part  of  the  antero-lateral  column,  conducting  the  crossed 

298 


THE  NERVOUS  SYSTEM 

conscious  muscular  sensations  and  passes  into  the  cerebellum  by 
the  superior  peduncle.  The  second  arises  in  cells  of  the  superior 
corpora  quadrigemina  and  passes  into  the  cerebellum  also  by  the 
superior  peduncles. 

Inasmuch  as  the  superior  corpora  quadrigemina  receive  the 
fibers  of  the  optic  nerve,  this  tract  must  transmit  association  im- 
pulses important  for  muscular  coordination  between  the  sense  of 
vision  and  the  cerebellar  centers,  an  association  much  used  in  many 
muscular  movements,  few  of  which  are  not  guided  by  sense  of 
vision.  So  much  for  the  afferent  tracts  of  the  cerebellum.  The 
efferent  fibers  to  the  cells  of  the  formatio  reticularis,  contained  in 
the  middle  peduncle,  have  been  mentioned. 

The  Efferent  Tracts  from  the  Cerebellum  —  All  efferent  fibers 
from  the  cerebellum  leave  from  the  central  nuclei,  the  nucleus 
dentatum,  fastigii,  globosus  and  emboliformis.  No  efferent  cere- 
bellar fibers  leave  the  cortex.  A  large  mass  of  fibers  leave  the 
nucleus  dentatum  and  pass  by  the  superior  peduncle  to  the  red 
nucleus  and  subthalamic  region  of  opposite  side.  A  certain  number 
of  fibers  pass  also  from  the  central  nuclei  to  the  corpora  quadri- 
gemina of  the  same  side.  No  fibers  pass  directly  to  the  spinal 
cord  but  important  tracts  run  between  the  central  nuclei  and 
the  nucleus  of  Bechterew  and  Deiters.  From  these  nuclei  large 
tracts  run  down  to  the  different  levels  of  the  spinal  cord  in  the 
antero-lateral  column  constituting  the  vestibulo-spinal  column. 
It  is  doubtless  in  part  through  this  tract  that  the  impulses  of 
equilibrium  are  capable  of  affecting  the  motor  apparatus  of  the 
spinal  cord,  passing  by  way  of  the  vestibular  nerve,  first  to  the 
cerebellum  where  they  become  modified  into  impulses  permitting 
finer  muscular  adjustments  by  association  with  other  impulses 
within  this  large  center  of  coordination  where  all  impulses  having 
to  do  with  muscular  contraction  meet. 

The  Trapezium  and  Lateral  Fillet  (see  Figs.  102  and  126)  — 
Two  other  tracts  of  white  matter  through  the  mid-brain  and  medulla 
are  yet  to  be  considered.  One  of  these  connects  the  nuclei  of  the 
auditory  nerve  with  the  inferior  corpora  quadrigemina.  From  both 
divisions  of  the  nuclei  of  the  auditory  nerve,  the  dorsal  and  ventral 
nucleus,  nerve  fibers  pass  internally  to  decussate  with  similar  fibers 
of  the  opposite  side  (Figs.  105  to  116.) 

This  decussating  tract  is  called  the  trapezium  and  forms  a  defi- 

300 


THE  NERVOUS  SYSTEM 

nite  structure  in  the  medulla.  The  trapezium  is  situated  just 
dorsal  to  the  formatio  reticularis.  It  is  joined  by  nerve  fibers 
from  the  superior  olive  and  by  the  fibers  of  the  striae  acusticse 


Fig.  126. 

A.  Auditory    fibres   passing   by    way    of    the    stria    acustica,    1,   and   the 
trapezium,  2,  and  the  lateral  fillet  to  the  inferior  corpus  quadrigeminum,  3. 

B.  Vestibular  fibres  after  making  connections  through  the  medulla  pass- 
ing to  the  dentate  nucleus,  4. 

C.  Optic  fibres  passing  to  the  superior  corpus  quadrigeminum,  5,  from 
which  fibres  run  to  the  cerebellar  cortex,  6,  and  posterior  longitudinal  bundle, 
7,  which  in  turn  establishes  connections  with  the  III  and  IV  nucleus  and  the 
Vlth  and  the  anterior  horn  cells,  8,  by  means  of  the  antero-lateral  column,  9. 

D.  Afferent  cerebellar  fibres  composed  of  the  posterior  cerebellar  tract,  10, 
to  the  cerebellar  cortex,  6,  and  the  superficial  external  arcuate  fibres,  11,  to 
the  cortex  of  the  vermis.    The  direct  cuneate  cerebellar  fibres,  12,  and  the 
olivo-cerebellar  fibres,  13. 

E.  Efferent   cerebellar  fibres  to  the   red  nucleus,   14,  from  the   dentate 
nucleus,  4. 

302 


THE  NERVOUS  SYSTEM      - 

after  these  have  crossed  on  the  floor  of  the  medulla  and  fibers  from 
the  auditory  nucleus.  All  these  fibers  become  collected  into  a 
bundle  which  passes  upwards  through  the  mid-brain  where  they 
form  the  lateral  fillet.  It  lies  to  the  outer  side  of  the  superior 
cerebellar  peduncle.  It  virtually  passes  around  the  peduncle  on 
its  outer  side  and  in  this  manner  gains  the  inferior  corpora  quad- 
rigemina  in  which  the  fibers  of  the  lateral  fillet  end.  (Figs.  105- 
116.)  The  inferior  corpora  quadrigemina  form  substations  for 
auditory  sensations. 

The  Posterior  Longitudinal  Bundle  —  The  other  white  tract 
through  the  mid-brain  and  pons  is  the  posterior  longitudinal  bundle. 
It  is  an  important  bundle  seen  in  all  sections  through  the  pons  and 
mid-brain,  and  continued  throughout  the  spinal  cord  in  the  antero- 
lateral  column  as  the  tract  of  Marie.  (Figs.  126  and  105-116.) 

The  posterior  longitudinal  bundle  is  a  well-defined  tract  run- 
ning near  the  middle  line  just  dorsal  to  the  tegmentum  in  the 
mid-brain,  and  to  the  formatio  reticularis  in  the  medulla  oblongata. 
It  connects  the  nuclei  of  the  various  cranial  nerves  with  each 
other  and  contains,  therefore,  fibers  which  run  in  both  directions. 

Summary  of  the  Various  Substations  —  We  have  now  consid- 
ered the  principal  new  masses  of  gray  matter  which  have  been 
added  to  the  cerebrospinal  axis  in  the  hind  and  middle  brain. 

We  have  also  followed  the  connections  of  these  nuclei,  and  the 
principal  tracts  connecting  these  nuclei  and  carrying  impulses 
from  them  and  from  the  spinal  cord  up  to  them:  they  may  be 
termed  the  terminal  substation  of  impulses,  standing  next  to  the 
cerebrum  in  the  receipt  or  transmission  of  impulses  passing  be- 
tween the  cerebrum  and  the  lower  portions  of  the  nervous  system. 
These  terminal  substations  are  situated  at  different  levels  for 
various  white  tracts  in  the  cerebrospinal  axis.  In  the  case  of  the 
pyramidal  tracts  the  terminal  substation  between  the  cerebrum 
and  the  spinal  cord  is  in  the  spinal  cord  itself.  In  the  case  of 
other  fibers  also  running  in  the  crura  cerebri  the  terminal  sub- 
station is  in  the  formatio  reticularis.  For  other  tracts  it  is  in  the 
red  nucleus  and  the  corpora  quadrigemina,  while  in  the  case  of 
still  others  the  terminal  station  is  in  the  base  of  the  fore-brain  itself, 
namely  in  the  optic  thalami. 

The  New  Masses  of  Gray  Matter  Belonging  to  the  Fore-Brain 
—  It  now  remains  to  describe  the  new  masses  of  gray  matter  be- 

304 


THE  NERVOUS  SYSTEM 


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306 


THE  NERVOUS  SYSTEM 

longing  to  the  cerebrum  arising  from  what  we  have  termed  the 
precerebral  vesicles,  and  those  paths  of  connection  between  them 
and  the  so-called  terminal  substations  placed  between  the  cerebrum 
and  the  lower  portions  of  the  cerebrospinal  axis. 


Lateral  rentricle. 
Caudate  nucleus.  I 
Anterior  commissure.        '- 
Lentiform  nucleus. 
External  capsule. 
Clanstrum 


Savum  septi.  pellucidl. 
Septum  pellucidum. 
Corpus  callosum. 

Internal  capsule. 


Insula. 


Middle  cerebral  artery. 


Anterior  per- 
I       forated  space. 
I      Optic  tract. 

Internal  carotid  artery.  I    Lamina  terminalis. 

Optic  recess.  Third  nerve. 


Fig.  128. — View  from  the  front  of  a  coronal  section  of  an  adult  brain  made 
two  and  a  half  inches  behind  the  frontal  pole  and  nearly  one  inch  behind 
the  temporal  pole  and  about  half  an  inch  posterior  to  the  anterior  end  of 
the  lateral  ventricles.  Five-sixths.  (Quain.) 

The  vallecula  Sylvii  is  seen  on  each  side  external  to  the  optic  commissure; 
on  the  right  side  of  the  brain  the  internal  carotid  artery  is  shown  dividing 
in  this  space  into  the  anterior  and  middle  cerebral  arteries. 


The  majority  of  new  masses  of  gray  matter  of  the  fore-brain 
have  already  been  described.  They  include  the  cerebral  cortex 
itself  and  the  walls  of  the  third  ventricle,  the  structures  appear- 
ing in  the  lateral  ventricles,  the  caudate  nucleus  and  the  optic 
thalami  themselves. 

308 


THE  NEEVOUS  SYSTEM 


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312 


THE  NERVOUS  SYSTEM 


The  Internal  Capsule  —  Internal  to  the  optic  thalami  is  the 
third  ventricle,  the  lateral  walls  of  which  are  formed  by  the  optic 
thalami.  A  small  portion  of  the  optic  thalami  appears  in  the 
lateral  ventricle.  Most  of  its  external  surface  is  surrounded  by 
thick,  capsule-like  layer  of  white  fibers  termed  the  internal  capsule, 
the  great  pathway  of  all  afferent  and  efferent  impulses  to  and 
from  the  cerebrum. 

The  internal  capsule  is  formed  of  all  those  nerve  fibers  which 
pass  from  various  portions  of  the  cerebrum  in  the  crura  cerebri, 
and  in  part  of  fibers  emerging  from  the  optic  thalami  to  be  dis- 


Induseum. 


Commissura  hippocampi.       Gyrus  cinguli.  j 


Stria  longitudinalis  medialis. 

^  Cavum  septl  pellucidi. 
^f  Septum  pellucidum. 

Ventriculus 
"P.      lateralis. 

Crus  fornicis. 


Plexus 

chorioideus 
lateralis. 

Stria  terminalis. 


Tela   chorioidea. 


Attachment  of  lamina 

chorioidea. 
t  -  Thalamus  (free  surface). 

Tseuia  thalami. 
Plexus   chorioideus  vent,   tertii.         Ventriculus  tertius 


Thalamus. 


Fig.  131. — Diagram  of  transverse  section  across  the  central  parts  of  the  lateral 
ventricles.     (Cunningham.) 

tributed  to  many  parts  of  the  cerebrum.  External  to  the  internal 
capsule  is  another  nucleus  of  gray  matter  termed  the  lenticular 
nucleus.  It  is  quite  a  large  nucleus,  shaped  somewhat  like  a  bicon- 
vex lens  on  both  transverse  and  horizontal  section.  It  separates 
the  internal  capsule  from  another  layer  of  white  fibers  termed  the 
external  capsule.  Outside  the  external  capsule  is  another  nucleus 
of  gray  matter,  thin  on  transverse  section,  termed  the  claustrum. 

Classification  of  the  Cerebral  Nerve  Tracts  —  The  tracts  of 
white  fibers  of  the  cerebrum  may  be  classed  as  nerve  tracts  con- 
necting the  brain  with  lower  levels.  They  are  afferent  and  efferent. 

Nerve  tracts  connecting  different  portions  of  one  cerebral 
hemisphere. 

Tracts  connecting  two  cerebral  hemispheres. 

The  Afferent  Tracts  of  the  Cerebrum  —  The  thalamo-cortical 

314 


THE  NERVOUS  SYSTEM 


THE  NERVOUS  SYSTEM 


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320 


THE  NERVOUS  SYSTEM 

Central  fissure. 
Posterior  central  gyrus. 


Corpus  callosum. 


Fornix. 
Anterior  central  gyrus.  /   /      Later&1  ventride 

Superior  frontal  /  I   /  -  Thalamus 

Sylvian  fissure.  gyrus.          /  /   /  /    .  Caudate  nucleus. 

' Internal  capsule. 
Middle  frontal 
gyrus. 


Claustrum. 

Inferior  horn   of 

lat.    vent. 
Hippocampal   fissure. 

Optic  tract. 
Hippocampal  gyrus. 
Uncus. 
Cerebral  peduncle. 

Pons. 

Insula.  \ 

Second  temporal  gyrus.  Pyramid  of  medulla  oblongata. 

First  temporal  gyrus. 

Fig.  135. — View  from  the  front  of  a  coronal  section  of  an  adult  brain  made 
three  inches  behind  the  frontal  pole.    Five-sixths. 

tracts.  From  all  parts  of  the  optic  thalami  fibers  stream  out  into 
the  internal  capsule  to  carry  on  the  impulse  arriving  at  the  optic 
thalami  to  all  parts  of  the  cerebral  cortex.  Entering  the  internal 

322 


THE  NERVOUS  SYSTEM 

capsule  these  fibers  may  be  divided  into  a  frontal,  parietal,  occip- 
ital and  temporal  group. 

The  frontal  fibers  are  contained  in  the  anterior  limit  of  the 
internal  capsule  and  run  to  the  frontal  lobe.  Some  of  the  fibers 
pass  to  the  lenticular  nucleus  and  then  other  axons  carry  on  the 
impulses  through  the  external  capsule. 

The  parietal  fibers  issue  from  the  lateral  surface  of  the  optic 
thalami  and  pass  to  the  parietal  lobe  through  the  middle  portions 
of  the  internal  capsule.  The  occipital  fibers  form  radiations  called 
the  occipital  radiations,  and  pass  to  the  occipital  lobe  through  the 
posterior  portion  of  the  internal  capsule,  coming  chiefly  from  the 
pulvinar  or  posterior  tubercle  of  the  optic  thalamus  and  the 
external  geniculate  body. 

The  fibers  issuing  from  the  under  surface  of  the  optic  thalami 
pass  under  the  lenticular  nucleus  to  the  temporal  lobe  and  island 
of  Reil.  Some  of  these  fibers,  known  as  the  auditory  radiations, 
pass  directly  into  the  "posterior  portion  of  internal  capsule  from 
the  internal  geniculate  body. 

Functions  of  the  Thalaino-cortical  Fibers  —  Of  these  various 
afferent  fibers,  those  passing  to  the  parietal  lobes  carry  onward  to 
the  cortex  the  cutaneous  and  possibly  muscular  sensations,  reach- 
ing the  optic  thalami  through  the  mesial  fillet.  Also  among  the 
fibers  of  the  anterior  or  middle  portions  of  the  internal  capsule 
are  those  carrying  on  the  impulses  reaching  the  red  nucleus  and 
optic  thalami  by  the  superior  cerebellar  peduncles. 

They  must  be  considered  as  furnishing  information  regarding 
the  fine  adjustments  in  muscular  coordination  being  produced  by 
the  cerebellum.  The  optic  radiations  carry  visual  impulse  from 
the  terminations  of  the  optic  nerve  in  the  superior  corpora  quad- 
rigemina.  By  other  fibers  between  the  occipital  cortex  and  these 
basal  nuclei  impulses  may  pass  from  the  cortex  to  the  superior  cor- 
pora quadrigemina  and  thence  as  afferent  impulses  to  the  cere- 
bellum. Through  the  auditory  radiations  to  the  temporal  lobes 
are  transmitted  impulses  from  the  inferior  corpora  quadrigemina 
and  internal  geniculate  body,  which  nuclei  receive  the  termination 
of  the  lateral  fillet  directly  from  the  auditory  nucleus. 

Other  fibers  pass  between  the  optic  thalamus  and  the  cerebral 
cortex  by  way  of  the  corpus  striatum.  The  larger  portion  of  the 
afferent  fibers  of  this  body  come  from  the  optic  thalami.  Other 

324 


-  Fibrse  propriae. 

-  Pyramidal  fibre. 

_  .-Corona  radiata. 


Ganglia  of  sensory  cranial  nerves. 


.      A! 

Nucleus   of   spinal   tract   of   trigemluus.  _\ 


"    Internal  capsule. 

Hypothalamic  nucleus. 


Fibres    to    hypothalamic  \ 
nucleus  of  same   side. 


Nuclei  of  termination  of! 
sensory  cranial  nerves.  \ 


_  _  Nucleus  of  fasciculus 
cuneatus. 

Nucleus  of  fasciculus 

gracilis. 


Posterior  root. 
Spinal  ganglion. 


_  Fasciculus  cuneatus. 
Fasciculus  gracilis. 


rrg.    136. — Scheme    of    ascending    or    spinocerebral    conduction    pathways.     (Morris.) 

326 


THE  NERVOUS  SYSTEM 


Somsesthetic  area 
of  cerebral  cor- 
tex. 

Caudate  nucleus. 

Internal  capsule. 

Lenticular  nucleus. 

Cerebral  peduncle. 
Trochlear  nerve. 


Medulla    oblongata 


Motor 
nuclei  of 
cranial 
nerves. 


Oculo-" 
motor. 

Masti- 
cator. 


Abducens. 

Facial. 

Glossopharyngeal. 

Vagus. 

Accessory. 

Hypoglossal. 


Decussation  of  pyramids. 


Lateral  cerebrospinal 

fasciculus. 
Ventral  cerebrospinal 

fasciculus. 


Ventral  roots  of  spinal  nerves. 
Ventral  white  commissure. 


Spinal  cord. 

«'ig.  137.— Scheme  of  descending  cerebrospinal  conduction  pathways.    (Morris.) 


328 


THE  NERVOUS  SYSTEM 

fibers  arising  in  the  corpus  striatum  pass  in  the  dorsal  part  of  the 
crusta  to  the  nuclei  pontis  of  the  formatio  reticularis. 

Connections  undoubtedly  exist  in  both  directions  between  the 
cortex  cerebri  and  the  nuclei  of  the  corpus  striatum.  The  chief 
nucleus  of  the  corpus  striatum  is  the  caudate  nucleus.  The  len- 
ticular nucleus  and  claustrum  form  similar  connections. 

Pyramidal  Tracts  —  The  efferent  tracts  from  the  Cerebrum  — 
The  pyramidal  tract  originates  as  axis  cylinders  of  the  cells  in 
the  ascending  frontal  convolution.  The  fibers  pass  downwards  and 
inwards  through  the  white  matter  of  the  hemispheres  to  the  in- 
ternal capsule.  In  this  structure  they  occupy  the  middle  two- 
fifths  on  transverse  section.  The  internal  capsule  presents  a  bend 
at  the  juncture  of  the  anterior  one-third  and  posterior  two-thirds, 
with  the  concavity  outwards,  and  surrounds  the  lenticular  nucleus. 
Anterior  to  the  bend  the  caudate  nucleus  lies  internal  to  it  and 
posterior  to  the  bend  the  optic  thalamus  lies  internal  to  it.  The 
pyramidal  fibers  occupy  the  bend  and  the  anterior  two-thirds  of 
the  portion  posterior  to  the  bend.  In  this  portion  the  fibers  con- 
trolling the  muscles  of  the  head  lie  anteriorly,  then  the  fibers 
belonging  to  the  anterior  extremity,  the  trunk  and  posterior  ex- 
tremity. The  pyramidal  fibers  finally  leave  the  internal  capsule 
and  enter  the  crusta  or  crura  cerebri.  These  structures  may  be 
viewed  as  the  stems  of  the  brain.  In  the  mid-brain  they  lie  ventral 
to  the  rest  of  the  mid-brain  and  diverge  as  they  are  traced  upwards 
and  forwards  between  the  mid-brain  and  the  cerebrum,  to  enter  the 
latter  by  forming  the  internal  capsule.  The  two  crusta  thus  fork 
to  inclose  between  them  as  they  enter  the  brain  the  two  optic 
thalami.  (Fig.  137.) 

The  remainder  of  the  mid-brain,  that  is  the  dorsal  portion, 
enters  the  cerebrum  by  passing  directly  into  the  optic  thalami. 
In  the  crusta  the  pyramidal  fibers  form  the  middle  two-fifths  of 
that  structure. 

A  small  portion  of  the  upper  part  of  the  external  surface  of 
the  optic  thalami,  above  the  diverging  fibers  of  internal  capsule, 
lies  free  in  the  beginning  of  the  descending  horn  of  the  lateral 
ventricle. 

Fronto-pontine  Fibers  —  In  the  anterior  limb  of  the  internal 
capsule  other  nerve  fibers,  arising  as  axis  cylinders  of  the  nerve 
cells  in  the  frontal  lobes  of  the  brain,  pass  down  into  the  mesial 

330 


THE  NERVOUS  SYSTEM 

portion  of  the  crusta  and  end  around  the  scattered  cells  in  the 
formatio  reticularis. 

Temporo-pontine  Fibers  —  Other  efferent  fibers  from  the  cere- 
brum arise  in  the  temporal  lobes  and  reach  the  posterior  limb  of 
the  internal  capsule  by  passing  under  the  lenticular  nucleus.  They 
then  reach  the  external  division  of  the  crusta  and  end  in  the  scat- 
tered cells  of  the  pons. 

Both  the  fronto-pontine  and  temporo-pontine  fibers  represent 
efferent  tracts  from  the  cerebrum  and  afferent  to  the  cerebellum 
of  the  cerebro-cerebellar  connections,  being  continued  into  the 
lateral  hemispheres  of  the  cerebellum  by  the  transversely  running 
middle  peduncles.  The  return  tract  of  this  cerebro-cerebellar  con- 
nection is  from  the  cortex  of  cerebellum  to  the  dentate  nucleus, 
then  by  the  superior  peduncles  to  the  red  nucleus  and  optic  thalami, 
and  finally  from  the  optic  thalami  to  the  cerebral  cortex.  Part 
of  the  thalamo-cortical  fibers  have  already  been  described,  those 
passing  directly  into  the  internal  capsule  and  around  the  lenticular 
nucleus  as  the  thalamo-frontal,  thalamo-parietal  and  the  auditory 
and  optic  radiations. 

Intra-Cerebral  Association  Tracts  —  Short  and  long  association 
tracts  exist  within  the  cerebrum.  The  short  tracts  pass  in  U-shaped 
loops  between  the  various  convolutions  around  the  bottom  of  the 
sulci.  The  long  tracts  may  be  divided  into  longitudinal  tracts  and 
commissural  tracts.  The  longitudinal  tracts  are: 

Uncinate  fasciculus  —  Between  the  orbital  convolutions  of  the 
frontal  lobes  and  the  front  part  of  the  temporal  lobes,  around  the 
bottom  of  the  fissure  of  Sylvius. 

The  cingulum  —  From  the  anterior  perforated  space  over  the 
dorsum  of  the  corpus  striatum  to  the  hippocampus  major  and  an- 
terior part  of  temporal  lobe.  (Fig.  138.) 

Superior  longitudinal  fasciculus  —  Somewhat  the  same  course 
as  the  cingulum,  connecting  the  frontal  parietal  and  occipital 
lobes. 

Inferior  longitudinal  fasciculus  —  External  to  the  optic  radia- 
tion between  the  temporal  and  occipital  lobes. 

Occipito-frontal  fasciculus  —  Runs  close  to  caudate  nucleus  in 
outer  walls  of  lateral  ventricle. 

The  commissural  fibers  include : 

(a)  The  great  mass  of  cortical  fibers  running  between  the 

332 


THE  NERVOUS  SYSTEM 


334 


THE  NERVOUS  SYSTEM 

two  hemispheres  and  constituting  the  major  portion  of  the  corpus 
callosum. 

(b)  The  anterior  commissure,  connecting  the  two  olfactory 
lobes  and  portions  of  the  two  temporal  lobes. 

(c)  Middle  commissure,  between  the  optic  thalami. 

(d)  The  hippocampal  commissure,  a  thin  lamina  between  the 
diverging  posterior  pillars  of  the  fornix,  appears  on  the  under 
surface  of  the  corpus  callosum,  connecting  the  two  hippocampi 
inajora, 

FUNCTIONS  AND  CONNECTIONS  OF  THE  CRANIAL  NERVES 

Olfactory  Nerves  —  The  olfactory  as  the  optic  nerves  are  to  be 
viewed  as  cerebral  associated  tracts  connecting  the  brain  with  a 
more  distal  portion  of  this  same  organ. 

Anatomically  they  are  different  from  the  other  cranial  nerves. 

The  olfactory  nerve  fibers  are  derived  from  cells  situated  upon 
the  surface  of  the  body  imbedded  within  the  nasal  mucous  mem- 
brane. One  process,  the  real  olfactory  nerve,  passes  forward  toward 
the  surface  to  end  in  the  olfactory  end  sense  organ.  The  other 
process  passes  backward  as  a  medullated  nerve  fiber,  through  the 
cribriform  plate  to  the  olfactory  bulb,  where  they  terminate  in  a 
terminal  arborization  among  the  branches  of  another  terminal 
arborization  of  the  peripheral  process  of  another  nerve  cell  called 
a  mitral  cell. 

It  is  the  axons  of  the  mitral  cells  which  form  the  olfactory 
tracts.  Each  tract  divides  posteriorly  into  two  roots,  a  mesial 
root  ending  in  the  anterior  end  of  the  callosal  gyrus  of  the  limbic 
lobe,  and  a  lateral  root,  crossing  the  anterior  perforated  space  to 
end  in  the  uncinate  extremity  of  the  hippocampal  gyrus.  Between 
these  two  tracts  is  a  prominence,  the  olfactory  tubercle. 

Portions  of  the  Brain  Forming  the  Olfactory  Mechanism  — 
The  following  portions  of  the  brain  serve  as  central  nuclei  and 
association  tracts  of  the  olfactory  apparatus. 

(1)  Olfactory  bulb  and  tract. 

(2)  Anterior  perforated  space. 

(3)  Anterior  portion  of  the  uncinate  gyrus. 

(4)  Septum  lucidum. 

(5)  Hippocampal  convolution. 

336 


THE  NERVOUS  SYSTEM 

(6)  Anterior  commissure. 

(7)  Trigonum  habenulae. 

(8)  Fornix. 

(9)  Corpora  mammillaria. 

(10)  The  bundle  of  Vicq  d'Azyr. 

(11)  Optic  thalami. 


Fig.  139. — Diagram  of  the  principal  components  of  the  optic  apparatus. 

(Morris.) 

The  Optic  Nerve  —  The  real  optic  nerves  are  merely  the  short 
processes  of  nerve  cells,  situated  within  the  retina  and  passing 
to  the  sensory  epithelium  of  the  retina.  The  central  processes  of 
these  nerve  cells  pass  in  the  opposite  direction  and  form  the  optic 
tracts.  These  partially  decussate  in  the  optic  chiasma  in  such  a 

338 


THE  NERVOUS  SYSTEM 


manner  that  only  the  fibers  from  the  internal  half  of  each  retina 
cross.  The  optic  tracts  end  posteriorly  in  the  pulvinar  of  the 
optic  thalami,  the  external  geniculate  body  and  the  superior  cor- 
pora quadrigemina.  From  these  connections  nerve  fibers  enter  the 
posterior  portion  of  the  internal  capsule  and  pass  by  the  optic 
radiations  to  the  occipital  lobes.  This  nerve  transmits  visual 
sensations. 


MUM.  L  AT.  ANT 
(0*RKSCMeW!TSCH) 


NUCl.OORS.I.(Ufl9 

WOCL.VENT.I.  (ANT.)—.. 


•NUCk.OOR6.ll.(>'OSl$ 
(V.GVDOSN) 


NUCU.CEHTRWJS- 

;NUCUVerr.li.(posT.) 


Fig.  140. — Diagram  of  the  groups  of  cells  forming  the  nuclei  of  the  third 

and  fourth  nerves.     (Quain.) 

The  fibres  from  the  nucleus  of  Darkschewitsch  to  the  oculo-motor  nerve 
are  doubtful. 

The  Third,  Fourth  and  Sixth  Nerves  —  The  third  and  fourth 
and  sixth  nerves  may  be  regarded  as  arising  from  one  continuous 
elongated  nucleus,  extending  from  the  level  of  the  striae  acusticae 
in  the  fourth  ventricle  to  the  superior  corpora  quadrigemina,  close 
to  the  middle  line. 

The  anterior  portion  of  this  nucleus,  that  belonging  to  the 
third  nerve,  may  be  divided  into  different  portions,  a  more  lateral 
large-cell  portion,  a  superficial  small-cell  portion  and  another 
median  portion  of  large  cells.  (Figs.  140  and  141.) 

Stimulation  from  before  backward  beginning  at  the  posterior 

340 


THE  NERVOUS  SYSTEM 

boundary  of  the  third  ventricle  gives  contraction  of  the  ciliary 
muscle  (changing  the  curvature  of  the  lens  of  the  eye),  contraction 
of  the  pupil,  of  the  internal  rectus,  the  superior  rectus,  the  levator 
palpebras  superioris,  the  inferior  rectus  and  the  inferior  oblique. 
Finally,  when  the  nucleus  of  the  fourth  nerve  is  reached,  we  obtain 
contraction  of  the  superior  oblique,  and  when  the  sixth  nerve  is 
reached,  contraction  of  the  external  rectus. 


Fig.   141. — Section  through  the  upper  part  of  one  of  the  anterior  corpora 

quadrigemina  and  the  adjacent  part  of  the  thalamus.  (Quain.) 
s.,  aqueduct;  gr.,  gray  matter  of  aqueduct;  c.  q. s.,  quadrigeminal  emi- 
nence, consisting  of:  1.,  stratum  lemnisci;  o.,  stratum  opticum;  and  c.,  stratum 
cinereum;  Th.,  thalamus  (pulvinar) ;  c.g.i.,  c.g.e.,  internal  and  external 
(mesial  and  lateral)  geniculate  bodies;  br.s.,  br.i.,  superior  and  inferior 
brachia;  /.,  fillet;  p.L,  posterior  (dorsal)  longitudinal  bundle;  r.,  raphe;  ///, 
third  nerve;  nlll,  its  nucleus;  I. p. p.,  posterior  perforated  space;  s.n.,  sub- 
stantia  nigra;  above  this  is  the  tegmentum  with  its  nucleus,  the  latter  being 
indicated  by  the  circular  area;  cr.,  crusta;  11,  optic  tract;  M.,  medullary 
centre  of  the  hemisphere;  n. c.,  nucleus  caudatus;  st.,  stria  terminalis. 

All  these  nuclei,  as  also  the  endings  of  the  optic  nerves  in  the 
superior  corpora  quadrigemina,  are  connected  by  means  of  the 
posterior  longitudinal  bundle,  so  that  means  are  provided  for 
accurate  coordination,  not  only  between  the  oculo-motor  nuclei 
themselves,  but  because  many  of  the  fibers  of  the  posterior  longi- 
tudinal bundle  are  axons  of  cells  of  Deiters'  nucleus,  with  also 

342 


THE  NERVOUS  SYSTEM 

the  great  centers  of  coordination  of  the  whole  body  in  the  cere- 
bellum. 

The  important  tract  between  the  superior  corpora  quadrigemina 
and  the  cerebellum  indicate  that  the  connections  of  the  former 
with  the  optic  nerves  chiefly  serve  the  function  of  coordination  of 
visual  impulses  with  the  movements,  not  only  of  the  eye,  but  also 
of  the  rest  of  the  body. 

The  function  of  the  oculo-motor  nerves  is  not  entirely  motor. 
They  contain  a  large  proportion  of  afferent  fibers  from  the  muscle 
of  the  eye-ball.  After  total  desensitization  of  the  eye-ball  by 
cocaine  or  by  section  of  the  fifth  nerve,  the  movements  of  the 
eye-ball  will  be  carried  out  with  as  much  precision  as  under  normal 
conditions. 

The  Fifth  Nerve  —  The  fifth  nerve  is  loth  motor  and  sensory. 
It  supplies  sensation  from  the  whole  of  the  face  and  interior  of 
the  mouth.  Its  motor  fibers  supply  the  muscles  of  mastication. 
It  also  supplies  the  tensor  tympani  muscle.  It  contains  trqphic 
fibers. 

The  Seventh  Nerve  —  The  seventh  cranial  nerve  is  largely 
motor  to  the  muscles  of  the  face  and  some  of  the  internal  ear 
muscles.  Through  the  nervus  intermedius  of  "Wrisberg,  which  is 
usually  included  as  part  of  the  seventh  nerve  but  should  really 
be  considered  a  separate  nerve  containing  efferent  secretory  fibers 
to  the  submaxillary  and  sublingual  glands  and  afferent  fibers,  con- 
veying impulses  of  taste  and  general  sensibility  from  the  tongue 
backwards  from  the  geniculate  ganglion. 

The  Eighth  Nerve  —  The  eighth  nerve  possesses  two  definite 
functions.  Its  auditory  branch  carries  impulses  from  the  sensory 
auditory  epithelium.  Its  fibers  originate  in  the  bipolar  cells  of 
the  ganglion  of  Scarpa. 

The  nerve  enters  the  medulla  immediately  beneath  the  pons,  and 
terminates  in  its  dorsal  and  its  ventral  nucleus.  From  these  nuclei 
the  auditory  impulses  pass  to  the  brain  by  way  of  the  trapezium, 
the  lateral  fillet,  the  inferior  corpora  quadrigemina  and  the  audi- 
tory radiations.  The  vestibular  branch  originates  in  the  ganglion 
of  the  cochlea.  The  peripheral  processes  of  these  cells  end  in  the 
sensory  epithelium  of  the  ampulla  of  the  semicircular  canals  and 
of  the  saccule  and  utricle.  (Fig.  142.)  The  nerve  enters  the  me- 
dulla with  the  auditory  division  and  passes  to  the  dorsal  nucleus 

344 


THE  NERVOUS  SYSTEM 

fci  «4 

j      I 


346 


THE  NERVOUS  SYSTEM 

beneath  the  trigonum  acusticum.  It  makes  connections  with  the 
nucleus  of  Dieters  and  Bechterew.  Some  of  its  fibers  pass  directly 
to  the  cerebellum.  The  vestibular  never  transmits  only  sensations 
of  equilibrium.  Thus  it  is  that  its  most  important  connections  are 
with  the  cerebellum,  that  central  mass  of  gray  matter  the  chief 
function  of  which  is  to  preside  over  equilibrium.  Through  the 
vestibulo-spinal  tract  the  vestibular  impulses  affect  the  spinal 
centers. 

Through  the  posterior  longitudinal  bundle  the  nuclei  of  the 
third,  fourth  and  sixth  nerves  become  coordinated  with  the  ves- 
tibular impulses,  and  through  the  superior  cerebellar  peduncle 
by  way  of  the  red  nucleus  and  optic  thalamus  these  same  impulses 
reach  the  cerebral  cortex  and  excite  there  efferent  motor  impulses 
which  may  still  further  influence  the  motor  mechanism  of  the 
spinal  cord. 

The  Ninth  Nerve  —  The  ninth  and  tenth  cranial  nerves  are 
chiefly  sensory.  The  central  nuclei  form  one  continuous  column 
lateral  to  the  motor  nuclei  beneath  the  floor  of  the  fourth  ven- 
tricle. Both  these  nerves  contain  efferent  fibers  arising  from 
nuclei  internal  and  ventral  to  the  sensory  nuclei.  The  chief  motor 
nucleus  of  the  tenth  nerve  is  the  nucleus  ambiguus. 

The  following  are  the  functions  of  the  ninth  cranial  nerve: 

(1)  Motor  to  the  muscles  of  the  pharynx  and  base  of  the  tongue. 

(2)  Secretory  fibers  to  the  parotid  gland  by  way  of  the  optic 
ganglion.    (3)  Sensory  fibers  from  the  tongue,  mouth  and  pharynx. 
(4)  Inhibitory  fibers  to  the  respiratory  center. 

The  Tenth  Nerve  —  The  tenth  or  pneumogastric  nerve  is  the 
longest  nerve  in  the  body.  Its  connections  are  most  numerous 
and  its  functions  more  varied  and  important  than,  perhaps,  any 
other  single  nerve. 

Its  efferent  functions  are:  (1)  Motor  to  the  levator  paiati  and 
three  constrictors  of  pharynx.  (2)  Motor  to  larynx.  (3)  Inhib- 
itory to  the  heart.  (4)  Motor  to  muscles  of  esophagus,  stomach, 
and  small  intestines.  (5)  Motor  to  unstriated  muscle  in  the  walls 
of  the  bronchi  and  bronchioles.  (6)  Secretory  to  glands  of  stomach 
and  possibly  pancreas. 

Its  afferent  functions  are:  (1)  Regulate  inspiration,  accelerate 
•and  promote  inspiration  or  increase  expiration  as  in  coughing.  (2) 

348 


THE  NERVOUS  SYSTEM 

Depressor  and  pressor  from  heart  to  vasomotor  center.  (3)  Re- 
flex inhibition  of  the  heart. 

The  Eleventh  Nerve  —  The  eleventh  or  spinal  accessory  nerve 
should  not  be  considered  as  a  cranial  nerve.  Filaments  enter  it 
which  take  origin  by  series  of  roots  coming  from  cells  in  the  in- 
terior horn  of  cord  as  low  down  as  the  sixth  cervical  nerve  (spinal 
portion),  but  continuous  above  with  those  of  the  accessory  por- 
tion. It  is  a  purely  motor  nerve  to  the  trapezius  and  sterno- 
mastoid  muscle. 

The  Twelfth  Nerve  —  The  twelfth  cranial  nerve  arises  from 
cells  under  the  trigonum  hypoglossi  in  the  floor  of  the  lower  half 
of  the  fourth  ventricle.  It  is  purely  motor  in  function,  supplying 
the  muscles  of  the  tongue  and  those  muscles  attached  to  the  hyoid 
bone  and  the  extrinsic  muscles  of  the  larynx. 

THE  FUNCTIONS  OP  THE  VARIOUS  PORTIONS  OF  THE  BRAIN 

Methods  of  Study  —  Several  methods  are  available  for  investi- 
gating the  functions  of  the  brain. 

(1)  A  knowledge  of  the  anatomical  connections  of  the  tracts 
within  the  brain  furnishes  in  itself  information  upon  the  functions 
of  the  brain  which  is  second  to  none  in  importance.     It  is  for 
this  reason  that  a  detailed  study  of  the  anatomy  of  the  brain  has 
been  necessary. 

(2)  Considerable  information  upon  the  function  of  various 
portions  of  the  brain  is  available  from  a  study  of  the  differences  in 
the  histological  structure  of  the  brain. 

(3)  Direct  experimentation  by  isolation  or  ablation  of  por- 
tions of  the  brain  enable  us  to  know  the  function  of  the  portion 
operated  upon. 

(4)  The  study  of  the  symptoms  of  human  beings  affected  with 
tumors  or  other  diseases  of  the  brain  producing  its  destruction. 

The  function  of  any  portion  of  the  brain  must  depend  solely 
upon  the  efferent  tracts  which  leave  it  and  the  afferent  tracts 
entering  it. 

We  have  seen  that  the  animal  with  only  a  spinal  cord  is  a 
machine  for  the  performance  of  certain  reflexes.  The  reflexes 
involve  particularly  muscles  belonging  to  the  same  level  as  the 
stimulated  afferent  nerves.  They  are  inevitable  and  contain  no 

350 


THE  NERVOUS  SYSTEM 

incalculable  element.  The  frog  is  best  adapted  for  an  experi- 
mental investigation  of  such  a  character. 

If  we  investigate  in  this  manner  the  brain  we  begin  by  divid- 
ing the  bulb  by  a  section  between  the  medulla  and  pons.  Such 
an  animal  is  called  the  bulbo-spinal  animal.  It  will  present  cer- 
tain phenomena  not  present  in  the  spinal  animal  and  determined 
by  the  character  of  the  afferent  nerves  having  their  nuclei  in 
the  bulb. 

The  Afferent  Impulses  Received  by  the  Medulla  —  The  bulb 
receives :  (1)  Afferent  impressions  of  taste  from  the  tongue  through 
the  nervus  intermedius.  (2)  Through  the  ninth,  afferent  impres- 
sions from  the  tongue  and  pharynx.  (3)  Through  the  vagus  affer- 
ent impulses  from  the  whole  alimentary  canal  to  the  ileocolic 
sphincter  and  from  the  lungs  and  heart. 

Efferent  Impulses  Passing  from  the  Medulla  —  It  also  sends 
efferent  fibers  from  the  nucleus  ambiguus  to  the  larynx,  bronchi, 
esophagus,  stomach  and  intestines,  secretory  fibers  to  the  stomach 
and  inhibitory  fibers  to  the  heart. 

The  eighth  nerve  is  divided,  even  if  all  of  its  nucleus  is  not  by 
the  section.  The  twelfth  nerve  supplying  the  muscles  of  the 
tongue  is  preserved. 

The  Bulbo-Spinal  Animal  and  How  It  Differs  from  the  Spinal 
Animal  —  The  preservation  of  these  additional  centers  makes  it 
possible  for  the  bulbo-spinal  animal  to  maintain  those  visceral 
functions  which  are  under  the  nervous  control  of  the  bulb.  The 
blood  pressure  will  not  show  the  great  fall  present  in  the  spinal 
animal.  The  animal  will  also  continue  to  breathe  regularly  and 
its  heart  rate  will  remain  normal.  All  these  functions  may  be 
affected  by  appropriate  stimuli. 

There  is,  moreover,  a  certain,  though  ill  defined,  dependence 
of  the  skeletal  muscles  upon  the  visceral  functions;  so  that,  with 
the  preservation  of  the  visceral  nervous  control,  there  is  a  greater 
stability  in  the  response  of  the  bulbo-spinal  animal  to  reflexes. 

It  is  easier  to  evoke  movements  in  all  four  limbs.  The  key 
to  the  situation  is  the  preservation  of  visceral  functions  and  the 
nexus  between  these  and  the  skeletal  motor  functions.  The  mech- 
anism by  which  food,  including  oxygen,  is  seized,  tasted,  swallowed 
and  digested  and  its  distributions  in  part  controlled  is  preserved. 
If  in  the  frog  the  eighth  nerve  has  been  left  intact  a  certain  amount 

352 


THE  NERVOUS  SYSTEM 

of  the  sense  of  equilibrium  is  preserved.    The  animal  will  try  to 
right  itself  if  laid  on  its  back,  and  usually  succeeds. 

The  Pontine  Bulbo-Spinal  Animal  —  In  order  to  investigate 
these  the  brain  must  be  divided  by  a  section  at  the  upper  border 
of  the  pons.  The  motor  nuclei  of  the  fifth  and  sixth  nerves  have 
now  been  preserved,  as  have  the  lower  nuclei  of  the  organ  of  hear- 
ing  and  the  important  organ  of  static  sense,  the  nucleus  of  the 
vestibular  branch  of  the  eighth  nerve.  Such  an  animal  is  able  to 
walk,  spring  and  swim.  "When  placed  on  its  back  it  immediately 
turns  over  and  will  appreciate  the  rotation  of  a  turn-table,  when 
placed  upon  it,  by  turning  its  eyes  in  the  opposite  direction. 

The  controlling  influence  of  the  cerebellum  upon  the  independ- 
ently excessive  excitability  of  the  lower  centers  is  made  evident 
by  its  removal  from  the  pontine-spinal  animal.  If  after  section 
above  the  pons  the  cerebellum  is  also  removed,  the  animal  be- 
comes spontaneously  active,  crawling  about  until  blocked  by  some 
obstacle.  There  is  also  an  increased  activity  of  the  swallowing 
reflex,  anything  touching  the  mouth  is  snapped  at. 

In  the  mammal  there  is  a  similar  increase  of  reflex  activity, 
but  the  power  of  progression  is  not  retained. 

The  Animal  Possessing  the  Brain  and  Cord,  All  Below  the 
Upper  Level  of  the  Mid-Brain  —  The  Functions  of  the  Mid-Brain 
—  When  the  mid-brain  is  preserved  by  a  section  in  front  of  the 
anterior  corpora  quadrigemina,  the  animal  will  be  in  possession  of 
all  its  sensory  impressions  and  the  efferent  paths  to  all  the  eye 
muscles  except  the  olfactory  sense.  In  the  mammal  such  a  con- 
dition causes  ' ' decerebrate  rigidity."  The  limbs  are  held  more 
or  less  rigidly  in  a  position  of  extension  and  resist  passive  flexion. 
The  condition  would  appear  to  be  due  in  part  to  the  increased 
activity  of  the  lower  centers,  especially  of  the  cerebellum,  and  is 
reflex,  as  it  is  at  once  abolished  by  section  of  the  posterior  spinal 
roots,  and  in  part  to  the  removal  of  the  inhibitory  impulses  nor- 
mally flowing  through  the  cerebro-spinal  tracts.  It  must  be  re- 
membered the  pyramidal  tracts  in  the  frog  are  represented  by 
only  a  few  fibers. 

The  Animal  Possessing  the  Optic  Thalami,  All  the  Brain  and 
Cord  Below  Them  —  The  preservation  of  the  optic  thalami,  that 
is,  the  removal  of  the  cerebral  hemispheres  alone,  leaves  the  frog 
with  all  that  is  necessary  for  the  response  to  any  stimulus.  Un- 

354 


THE  NERVOUS  SYSTEM 

less  the  animal  is  observed  critically  one  would  fail  to  notice  any- 
thing wrong  with  the  animal.  It  sits  up  in  its  position,  on  inter- 
ference jumps  away,  guides  itself  perfectly  by  sight.  It  will 
swim  about  in  water  until  it  finds  a  support  upon  which  it  will 
crawl  out.  It  will  crawl  up  an  inclined  board  and  if  the  inclination 
is  gradually  increased  until  the  board  is  rotated  on  its  lower  end, 
the  frog  will  crawl  up  one  side  and  down  the  other. 

The  single  difference  between  such  an  animal  and  the  normal 
animal  is  the  entire  absence  in  the  former  of  spontaneous  motion. 
It  is  an  extremely  complex  and,  in  contrast  to  the  previously  de- 
scribed animal,  an  extremely  accurate  and  well-balanced  machine. 
Every  movement,  however,  must  be  provoked  by  a  closely  related 
external  stimulus. 

If  care  has  been  taken  to  preserve  the  optic  thalami  in  such  an 
animal  it  will  occasionally  show  spontaneous  movements,  such,  for 
instance,  as  attempts  to  bury  itself  as  if  to  hibernate  upon  the 
approach  of  winter.  If  the  optic  thalami  have  been  preserved  in 
fishes  they  show  very  little  difference  from  the  normal  fish. 

On  the  other  hand,  elasmobranch  fishes  which  depend  mainly 
upon  the  olfactory  apparatus,  the  removal  of  merely  the  olfactory 
lobes  and  cerebral  hemispheres  produce  almost  complete  paralysis, 
even  though  the  optic  lobes  and  thalami  are  intact. 

.  The  animal  contains  no  incalculable  element.  It  feels  no  hunger 
or  fear.  The  bird  acts  much  as  the  frog.  It  is  able  to  walk  about 
avoiding  obstacles  and  even  to  fly.  It  is  unable  to  recognize  food 
or  enemies  or  its  opposite  sex.  It  shows  no  fear. 

The  whole  of  the  cerebral  hemispheres  have  been  successfully 
removed  in  a  dog.  It  was  able  to  walk  normally  and  spent  most 
of  the  day  in  walking  up  and  down  its.  cage.  It  slept  soundly  at 
night.  In  pinching  its  skin  it  would  turn  around  and  snarl  and 
attempt  to  bite.  It  dould  not  recognize  food,  showed  no  fear  or 
pleasure  or  recognition  of  those  who  fed  it.  All  memory  was 
gone. 

Functions  of  the  Cerebellum  —  We  have  seen  that  there  are 
two  distinct  classes  of  afferent  stimuli;  we  may  speak  of  them  as 
two  systems  of  afferent  nerves. 

1.  Exteroceptive  or  stimuli  coming  from  the  surface  of  the 
body  or  striking  it  from  a  distance. 

2.  Proprioceptive  or  afferent  stimuli  from  the  interior  of  the 

356 


THE  NERVOUS  SYSTEM 


body,  the  muscles,  joints  and  tendons.  This  second  system  has  its 
head  ganglion  in  the  cerebellum.  By  its  afferent  nerves  it  fur- 
nishes information  as  to  the  exact  position  of  the  limbs  and  the 
degree  of  contraction  of  the  muscles.  As  a  part  of  this  system 
must  be  included  the  afferent  stimuli  entering  the  vestibular  branch 
of  the  eighth  nerve,  conveying  those  impressions  of  static  sense 
which  have  reference  to  the  body  as  a  whole. 

The  head  ganglion  of 
this  system  is  the  cerebel- 
lum. All  its  impressions 
are  received  and  properly 
balanced  against  one  an- 
other in  this  organ  and 
just  the  correct  efferent 
impulses  discharged  to  the 
higher  parts  of  the  central 
nervous  system,  but  also 
indirectly  to  the  spinal 
cord  through  the  vestibulo- 
spinal  tract  and  the  cere- 
bro-spinal  tracts,  to  pro- 
duce just  that  proper  de- 
gree of  relative  contrac- 
tion and  relaxation  of  op- 
posing sets  of  muscles 
which  will  result  in  a  per- 
fection of  coordination,  not 


Fig.    143.— Cells   of   the    cerebellar   cortex, 
showing  the   probable   path   of 

nerve-impulses.     (Quain.) 
A,  a  moss-fibre  (afferent) ;  B,  an  axon  of 
a  Purkinje  cell   (efferent);  a,  granules;   b, 
their  axons;  c,  a  Golgi  cell;  d,  two  Purkinje 
cells. 


only  in  the  normal  tone 
preserved  during  rest  but 
also  in  the  variations  of  contractions  incidental  to  the  muscular 
activities,  which  are  superimposed  by  the  higher  parts  of  the  cen- 
tral nervous  system  or  by  the  pure  reflexes  of  the  spinal  cord. 

The  Histology  of  the  Cerebellar  Cortex  —  The  cortex  of  the 
cerebellum  consists  of  the  following  two  layers  between  which  are 
situated  cells,  called  the  cells  of  Purkinje:  (Figs.  143  and  144.) 

1.  Molecular  layer  —  Most  external,  its  characteristic  cell  is 
star-shaped  with  an  axon  which  runs  parallel  with  the  surface. 
Prom  this  axon  collateral  fibers  dip  internally,  to  end  in  a  regular 
basket-like  arborization  around  the  cells  of  Purkinje. 

358 


THE  NERVOUS  SYSTEM 


2.     Granular  or  nuclear  layer  —  It  contains  two  kinds  of  cells : 

(a)     Small   cells   with   dendrites   and   one   axon   which   runs 

straight  up  into  the  molecular  layer  where  it  bifurcates  into  two 

branches  running  parallel  to  the  surface  and  resting  upon  the  tips 

of  the  tree  like  arborization  of 
Purkinje's  cells. 

(6)  Golgi's  cells  —  Cells  with 
many  dendrites  terminating  in 
the  neighboring  gray  matter. 

There  are  two  sets  of  affer- 
ent fibers  to  the  cerebellar  cor- 
tex and  one  set  of  efferent  fibers. 

1.  Moss      fibers  —  Afferent 
fibers  presenting  curious  thick- 
enings and  terminating  by  fre- 
quent branches  in  the  gray  mat- 
ter. 

2.  Tendril  fibers,  also  affer- 
ent    ending     by     arborization 
around  the  base  of  the  cells  of 
Purkinje. 

3.  Axons    of    the    cells    of 
Purkinje    run    down    into    the 
white  matter  to  end  around  the 
cells  of  the  deep  nuclei.    No  ef- 
ferent fiber  from  the  cortex  of 
the  cerebellum  leaves  the  cere- 
bellum. 

The  cells  of  Purkinje  are 
large,  flask-shaped  cells  with  one 
apical  dendrite  and  one  axon 


Fig.  144. — Section  of  cortex  of  cere- 
bellum.   (Quain.) 

a,  pia  mater;  b,  external  layer;  c, 
layer  of  corpuscles  of  Purkinje;  d, 
inner  or  granule  layer;  e,  medullary 
centre. 


from  the  base  of  the  cell.     The 
(H      one  dendrite  is  characterized  by 
the  richness  of  its  branching. 

The  Afferent  Tracts  to  the  Cerebellum  by  Way  of  the  Three 
Peduncles  —  The  afferent  tracts  of  the  cerebellum  are : 

Inferior  Peduncle —  (1)  Axons  of  Clark's  column  of  cells  by 
posterior  cerebellar  tract. 

(2)     From  the  nuclei  gracilis  and  cuneatus  of  the  same  and 
opposite  side. 

360 


THE  NERVOUS  SYSTEM 

(3)  From    vestibular    nerve    directly    and    indirectly    from 
Deiters'  nucleus. 

(4)  Inferior  olive  of  chiefly  the  opposite  side. 

Middle  Peduncle  —  Partly  afferent  and  partly  efferent  to  and 
from  the  formatio  reticularis.  By  means  of  the  nuclei  of  the 
formatio  re.ticularis  connections  are  established  between  the  frontal 
and  temporal  lobes  of  the  brain. 

Superior  Peduncle — (1)  From  the  superior  corpora  quad- 
rigemina  to  the  cortical  gray  matter,  and  thus  connections  are  es- 
tablished between  the  optic  nerve  and  oculo-motor  and  the  cere- 
bellum. 

(2)     The  anterior  cerebellar  tract  from  the  spinal  cord. 

The  Efferent  Tracts  from  the  Cerebellum  —  From  the  roof 
ganglia  the  impulses  from  the  termination  of  the  axons  of  the  cells 
of  Purkinje  are  passed  on  to  the  pons  by  the  middle  peduncle  and 
by  the  superior  cerebellar  peduncle  to  the  red  nucleus  and  subthal- 
amic  region.  Fibers  also  pass  from  the  roof  nuclei  to  the  superior 
corpora  quadrigemina. 

No  tract  runs  directly  from  the  cerebellum  to  the  cord,  but 
from  Deiters'  nucleus,  which  is  closely  connected  with  the  roof 
nuclei  of  the  cerebellum,  fibers  run  downward  to  the  cord  in  the 
vestibulo-spinal  tract. 

Muscular  movements  may  be  excited  by  stimulation  of  the 
cerebellum.  Unless  very  strong  stimuli  are  applied  to  the  cortex 
of  the  cerebellum  no  movements  are  excited.  It  is  not  likely, 
therefore,  that  any  of  the  cerebellar  efferent  fibers  leave  the  cortex. 
On  the  other  hand,  when  weak  stimuli  are  applied  to  the  central 
nuclei  movements  are  excited.  Stimulations  of  the  roof  nuclei 
will  produce  movements  of  the  eyes  and  head. 

Stimulation  of  the  nuclei  of  the  lateral  lobes,  the  nucleus 
dentatum,  will  produce  movements  of  the  trunk  and  limbs.  The 
movements  evoked  are  concerned  in  maintaining  equilibrium  and 
involve  every  muscle  of  the  body. 

Behavior  of  the  Cerebellarless  Animal  —  The  functions  of  the 
cerebellum  are  made  more  clear  by  removing  it  in  whole  or  in 
part.  Complete  unilateral  extirpation  of  the  cerebellum  leaves  the 
animal  (e.  g.,  the  dog)  with  three  cardinal  symptoms: 

(1)  Asthenia  or  loss  of  power  on  the  same  side  of  the  body. 

(2)  Atonia  —  considerable  loss  of  tone  on  the  same  side. 

362 


THE  NERVOUS  SYSTEM 

(3)  Astasia, —  tremors,  or  rhythmical  movements,  accompany- 
ing any  voluntary  movement.  A  dog  deprived  of  the  cerebellum 
upon  one  side  at  first  is  unable  to  stand  but  later  acquires  the 
power  to  walk,  though  the  hind  leg  drops  and  tremors  accompany 
every  movement.  The  animal  tends  to  fall  toward  the  side  of  the 
lesion.  It  attempts  to  support  itself  against  any  wall  or  support. 
When  the  whole  cerebellum  has  been  removed  the  animal  is  unable 
to  walk  for  months.  After  a  time  it  learns  to  do  so  but  shows 
an  ataxia  which  is  quite  different  from  spinal  ataxia  and  may  best 
be  described  as  a  top-heavy  ataxia.  It  is  an  ataxia  precisely  similar 
to  the  staggering  of  a  drunken  man.  The  compensation,  which  is 
acquired  after  cerebellar  lesions,  is  of  cerebral  origin.  Subsequent 
removal  of  the  hemispheres  produces  permanent  inability  to  walk. 
The  animal  or  human  being  without  a  functionating  cerebellum  is 
without  those  impulses  which  normally  constantly  stream  out  from 
it  in  response  to  the  afferent  impulses  of  the  proprioceptive  system 
and  either  directly  or  indirectly  reach  the  cord. 

In  attempting  to  furnish  an  explanation  of  the  symptoms  of 
the  cerebellarless  animal  a  number  of  possibilities  are  present,  all 
of  which  are  possible  factors.  We  have  in  the  first  place  not  merely 
an  interruption  of  a  large  portion  of  the  impulses  conveying  mus- 
cular sensations  and  of  proprioceptive  impulses  of  equilibrium 
through  the  vestibular  nerve  to  the  cerebellum,  but  also  a  loss 
of  what  is  of  greater  importance :  the  operations  of  the  mechanism 
which  gathers  up  all  the  afferent  impulses  expressing  the  state 
of  contraction  of  the  individual  muscles  and  the  relation  of  the 
center  of  gravity  of  the  whole  body  to  its  position,  and  which  sends 
out  as  its  response  to  these  incoming  muscular  impulses  a  constant 
call  upon  the  apparatus  of  the  more  peripheral  portion  of  the  ner- 
vous system  for  just  the  right  degree  of  contraction  and  relaxation, 
or  of  augmentation  and  inhibition,  which  affords  the  tone  of  rest 
and  the  steadying  of  the  changing  phases  of  muscular  activities. 

In  response  to  the  same  incoming  impulses  there  passes  to  the 
higher  portions  of  the  nervous  system  a  unified  call  as  though  from 
the  operations  of  a  clearing-house,  for  the  correct  adjustment  of 
voluntary  movement  to  the  preservation  of  the  desired  position  of 
the  center  of  gravity.  The  failure  in  this  apparatus  leaves  the 
animal  without  its  most  important  guide  in  the  adjustment  of  its 
movements.  In  its  absence  the  animal  must  fall  back  upon  the 

364 


THE  NERVOUS  SYSTEM 

cerebrum  which  is  incompletely  furnished  with  proprioceptive 
muscular  sensations  and  which  lacks  that  superlative  degree  of 
association  of  all  tracts  concerned  in  muscular  movements  which 
exists  in  the  cerebellum. 

Attempts  have  been  made  to  show  by  some  observers  that  the 
normal  activity  of  the  cerebellum  is  exerted  rather  on  the  side  of 
augmentation  of  muscular  function  (Starling)  and  by  others  on 
the  side  of  restraint  of  muscular  function  (Meyers,  J.  A.  M.  A., 
LXV,  16,  1348).  The  latter  idea  explains  better  the  condition  of 
decerebrate  rigidity  following  section  of  the  mid-brain  anterior  to 
the  superior  corpora  quadrigemina,  the  symptom  of  adiadokocinesis, 
and  the  spontaneous  activity  of  the  frog  after  removal  of  the  cere- 
bellum and  section  of  the  brain  in  front  of  the  pons. 

However  this  may  be,  the  greatest  emphasis  should  be  laid  on 
the  fact  that  the  supreme  function  of  the  cerebellum  is  the  exercise 
of  a  control  over  muscular  contraction,  which  control  has,  so  to 
speak,  for  its  aim  the  production  of  a  perfect  coordination  in  both 
states  of  rest  and  states  of  changing  muscular  activity. 

In  states  of  rest  coodination  is  provided  for  through  the  more 
direct  spinal  relations  of  the  cerebellum  and  during  voluntary 
muscular  activity,  no  impulse  may  descend  from  the  brain  without 
an  influence  upon  its  direction  being  imparted  to  it,  both  in  its 
incipiency  as  a  result  of  the  cerebrally  afferent  impulses  from  the 
cerebellum  and  in  its  course  as  a  result  of  the  lower  indirect  efferent 
cerebellar  connections  with  the  lower  spinal  centers. 

The  cerebellarless  animal  shows  three  principal  symptoms  which 
are  all  referable  to  the  same  side  in  unilateral  ablations : 

1.  Asthenia  —  a  slight  loss  of  power  or  weakness. 

2.  Atonia  —  or  loss  of  tone;  a  constant  undue  laxness  of  the 
muscles,  in  other  words. 

3.  Astasia  —  tremors  or  rhythmical  movements  of  the  muscles 
accompanying  every  willed  movement. 

All  these  three  symptoms  can  be  traced  to  the  failure  of  normal 
degree  of  responsiveness  of  the  muscles.  Each  contraction  starts 
in  a  more  relaxed  muscle  and  therefore  at  less  advantage,  and 
so  must  require  an  extra  voluntary  effort  to  accomplish  the  same 
end.  For  this  reason  there  is  apparent  weakness  of  the  muscles : — 
during  rest  an  undue  laxness  and  during  movement  a  frequent 
under  or  over  contraction,  which  results  in  tremors  and  inaccura- 

366 


THE  NERVOUS  SYSTEM 

cies  of  the  movement  to  the  desired  end.  This  inaccuracy  has 
its  origin  not  only  in  the  unpreparedness,  so  to  speak,  of  the 
muscles,  but  in  the  loss  of  the  organ  of  accurate  coordination. 

The  cerebellarless  animal  will  at  first  be  unable  to  stand  or 
walk,  but  after  several  months  may  again  be  able  to  walk.  The 
compensation  is  cerebral,  as  when  the  cerebrum  is  then  cut  off, 
permanent  paralysis  follows.  The  gait,  however,  of  such  a  cere- 
bellarless animal  is  characteristic.  There  is  a  constant  tendency, 
precisely  as  in  a  drunken  man,  for  the  center  of  gravity  to  fall  to 
one  or  -the  other  side.  It  is  a  top-heavy  ataxia.  The  animal  is 
ever  ready  to  take  advantage  of  a  wall  against  which  it  may  lean 
during  its  progression.  In  order  to  correct  this  tendency  for  the 
center  of  gravity  to  fall  to  one  side  or  the  other,  it  makes  its  base 
of  support  as  wide  as  possible.  Each  diagonal  movements  starts 
with  less  advantages  and  is  accomplished  with  less  accuracy  than 
under  normal  condition. 

There  is  then  a  tendency  for  the  feet  to  move  too  little,  in 
order  to  place  the  center  of  gravity  in  a  correct  position  for  bal- 
ancing. This  tendency  must  be  corrected  by  an  extra  and  usually 
an  inaccurate  effort  on  the  part  of  the  cerebrum,  so  that  exces- 
sive movements  are  made  which  cause  the  animal  to  adopt  a  wide 
base  of  support  and  to  stagger. 

This  form  of  ataxia  is  called  cerebellar  ataxia. 

It  must  be  distinguished  from  two  other  forms  of  ataxia:  (1) 
spinal  or  tabetic  ataxia,  accompanying  lesions  in  the  posterior 
columns  of  the  cord,  and  (2)  ataxia  due  to  lesions  in  the  pyram- 
idal tracts. 

In  spinal  ataxia  the  movements  are  excessive  and  inaccurate  be- 
cause the  cerebrum  is  not  furnished  with  information  as  to  the  de- 
gree of  contraction  within  the  muscles.  It  consequently  can  only 
know  the  results  of  its  efforts  by  the  use  of  the  eyes.  The  loss  of 
this  information  produces  the  impression,  so  to  speak,  on  the  cere- 
brum that  a  greater  degree  of  movement  is  required  than  the  cus- 
tomary amount  for  the  desired  end. 

In  addition,  therefore,  to  the  movements  being  inaccurate  they 
are  excessive.  The  cerebellar  and  vestibular  mechanism,  however, 
is  intact  and  so  there  is  not  that  loss  of  the  adjustment  of  move- 
ments of  the  body  as  a  whole  to  the  correct  position  of  the  center 
of  gravity  experienced  in  cerebellar  ataxia.  A  lesion  in  the  pos- 

368 


THE  NERVOUS  SYSTEM 

terior  columns  illustrates  defects  dependent  on  a  loss  of  one  kind  of 
function  presided  over  in  part  by  the  cerebellum. 

On  the  other  hand,  in  lateral  sclerosis,  in  which  condition  the 
lesion  is  in  the  pyramidal  tracts,  the  cerebrum  has  lost  the  main 
path  for  both  the  activation  and  control  of  the  motor  mechanism 
of  the  spinal  cord,  and,  in  consequence,  every  reflex  is  exaggerated. 


FUNCTIONS   OF   THE   CEREBRUM 

The  Contrast  between  the  Animal  Possessing  a  Cerebrum  and 
One  Without  a  Cerebrum  —  When  we  investigate  the  functions  of 
the  cerebrum  we  at  once  are  struck  with  a  very  important  difference 
between  the  animal  possessing  a  cerebral  hemisphere  and  one  with- 
out one.  An  animal  deprived  of  its  cerebral  hemispheres  can  be 
played  upon  at  will ;  a  definite  response  will  always  follow  a  definite 
stimulus.  With  an  animal  possessing  a  cerebral  hemisphere  it  is 
impossible  to  foretell  just  what  response  will  follow  any  stimulus. 
En  other  words  the  response  following  a  peripheral  stimulus  always 
is  incalculable.  Such  an  animal  is  influenced  by  many  feelings  in- 
volved in  the  action  of  its  consciousness.  Fear,  anger,  hunger,  affec- 
tion, will  all  cause  a  modification  of  stimulated  reflexes.  The  ques- 
tion suggests  itself,  and  did  long  ago  when  this  subject  was  first  con- 
sidered, are  certain  areas  in  the  cerebral  cortex  devoted  to  the 
exclusive  origination  of  these  various  impulses  or  feelings  which 
control  our  actions  ?  On  both  theoretical  and  experimental  grounds 
the  cerebral  cortex  cannot  be  thus  divided  into  areas  which  repre- 
sent the  various  predominating  states  which  characterize  our  con- 
sciousness. 

The  Foundation  of  all  Mental  Activity  upon  Association  of 
Ideas  (Association  of  Intracerebral  Groups  of  Impulses)  —  Abla- 
tion of  various  portions  of  the  brain  does  not  remove  any  one  form 
of  activity  characterizing  our  intelligence,  but  rather  induces  a 
reduction  of  intelligence  as  a  whole.  The  whole  science  of  phrenol- 
ogy has  no  basis  in  fact.  On  the  other  hand  certain  portions  of  the 
brain  are  intimately  associated  with  definite  forms  of  cerebral  activ- 
ity, but  only  with  those  forms  of  cerebral  activity  which,  if  we  may 
be  excused  for  using  the  term,  stand  immediately  next  to  the  out- 
side world  on  both  the  afferent  and  efferent  end  of  the  chain  of  links 
which  constitutes  perception,  judgment,  volition  and  finally  action. 

370 


THE  NERVOUS  SYSTEM 

The  afferent  and  efferent  end  of  this  chain  may  be  characterized  as 
perception  and  action. 

The  intervening  forms  of  cerebral  activity,  upon  which  judg- 
ment and  volition  are  based,  those  which  determine  what  action 
shall  follow  what  is  perceived,  call  into  activity  many  parts  of  the 
cerebral  cortex  and  are  only  possible  because  of  the  faculty  called 
memory.  But  what  is  memory  ?  Only  repetition  of  former  experi- 
ences within  the  mind,  the  actual  use  of  all  the  old  previously  well 
worn  cerebral  paths  used  in  former  experiences  minus  the  actual 
external  afferent  impulses  normally  associated  with  these  experi- 
ences. 

The  association  tracts  constitute  the  old  paths  and  the  old  ex- 
periences cannot  again  be  actuated  in  the  mind  without  the  use  by 
the  brain  of  all  the  association  tracts  in  the  brain  which  were  util- 
ized in  those  former  experiences,  and  by  again  bringing  into  rela- 
tion with  each  other  portions  of  the  brain  directly  connected  with 
perception  and  action,  though  these  portions  may  be  very  distant 
from  one  another.  And  so  it  is  that  all  the  brain  is  used  in  most 
of  our  intellectual  acts  and  states  of  consciousness. 

The  Localization  of  Function  in  the  Perceptive  and  Action  End 
of  the  Chain  of  Cerebral  Activities  —  All  in  the  above  paragraph 
is  very  far  from  meaning  that  no  portions  of  the  brain  are  definitely 
related  to  certain  forms  of  cerebral  activity.  It  is,  however,  only 
the  perception  and  action  end  of  the  chain  of  cerebral  events  which 
can  be  identified  with  special  areas  within  the  cerebral  cortex.  Let 
us  consider  first  the  areas  representing  action,  which  term  we  may 
select  to  describe  the  function  of  the  motor  areas.  The  motor  areas 
of  the  human  brain  are  all  within  the  ascending  frontal  convolution. 
From  above  downwards  are  centers  which  control  the  movements 
of  the  leg,  body,  arm  and  face.  (Figs.  145-147.) 

Stimulation  of  this  region  will  produce  coordinated  movements 
of  the  leg,  trunk,  arm  and  face.  In  the  dog  and  lower  animals  these 
areas  are  not  so  sharply  separated  as  they  are  in  the  higher  apes  or 
in  a  human  being.  Indeed  in  the  human  being  these  areas  are  even 
separated  by  unresponsive  spaces  or  partitions.  The  areas  may  be 
stimulated  by  weak  electrical  currents  directly  applied.  In  con- 
trast to  the  cerebellum  only  weak  currents  are  required,  even 
smaller  than  is  needed  to  stimulate  the  underlying  white  matter 
after  removal  of  the  gray  matter. 

372 


THE  NERVOUS  SYSTEM 


Abdomen 


Opening 

ffjdUtS 


Kxhl 

corcfs. 


Mastfcacton 


Fig.  145. — Brain  of  a  chimpanzee  (Troglodytes  niger).     Left  hemisphere  viewed  from  side 

and  above  so  as  to  obtain  as  far  as  possible  the  configuration  of  the  sulcus  centralis 

area. 

The  figure  involves,  nevertheless,  considerable  foreshortening  about  the  top  and  bottom 
of  sulcus  centralis.  The  extent  of  the  "motor"  area  on  the  free  surface  of  the  hemisphere 
is  indicated  by  the  black  stippling,  which  extends  back  to  the  sulcus  centralis.  Much  of 
the  "motor"  area  is  hidden  in  sulci;  for  instance,  the  area  extends  into  the  sulc.  centralis 
and  the  sulc.  precentrales,  also  into  occasional  sulci  which  cross  the  precentral  gyrus.  The 
names  printed  large  on  the  stippled  area  indicate  the  main  regions  of  the  "motor"  area. 
The  names  printed  small  outside  the  brain  indicate  broadly  by  their  pointing  lines  the  re- 
lation topography  of  some  of  the  chief  subdivisions  of  the  main  regions  of  the  "motor" 
cortex.  But  there  exists  much  overlapping  of  the  areas  and  of  their  subdivisions  which 
the  diagram  does  not  attempt  to  indicate. 

The  shaded  regions,  marked  "EYES,"  indicate  in  the  frontal  and  occipital  regions  re- 
spectively the  portions  of  cortex  which,  under  faradization,  yield  conjugate  movements  of 
the  eyeballs.  But  it  is  questionable  whether  these  reactions  sufficiently  resemble  those  of 
the  "motor"  area  to  be  included  with  them.  They  are  therefore  marked  in  vertical  shad- 
ing instead  of  stippling,  as  is  the  "motor"  area.  S.F.,  superior  precentral  sulcus.  LPr., 
inferior  precentral  sulcus.  (Sherrington.) 


374 


THE  NERVOUS  SYSTEM 


This  fact  demonstrates  that  the  cerebral  cortex  itself  and  not  the 
underlying  white  matter  is  being  stimulated.  The  fact  is  further 
attested  to  by  the  absence  of  the  power  to  stimulate  the  cortex  after 


SuU.Central. 


Sttlccattoso 
mon 

Sulc.parUCo 


Sulc.precentrmarg. 


Sulc.cdlcarin 


C.S.S.  del. 


Fig.  146. — Brain  of  a  chimpanzee  (Troglodytes  niger).  Left  hemisphere; 

mesial  surface. 

The  extent  of  the  "motor"  area  on  the  free  surface  of  the  hemisphere  is 
indicated  by  the  black  stippling.  On  the  stippled  area  "LEG"  indicates  that 
movements  of  the  lower  limb  are  directly  represented  in  all  the  regions  of 
the  "motor"  area  visible  from  this  aspect.  Such  mutual  overlapping  of  the 
minuter  sub-divisions  exists  in  this  area  that  the  diagram  does  not  attempt 
to  exhibit  them.  The  pointing  line  from  "Anus,  etc.",  indicates  broadly  the 
position  of  the  area  whence  perineal  movements  are  primarily  elicitable. 

Sulc.  central,  central  fissure;  Sulc.  calcarin.,  calcarine  fissure;  Sulc.  parieto 
occip.,  parieto-occipital  fissure;  Sulc.  calloso  marg.,  calloso-marginal  fissure; 
Sulc.  precentr.  marg..  pre-central  fissure. 

The  single  italic  letters  mark  spots  whence,  occasionally  and  irregularly, 
movements  of  the  foot  and  leg  (ff),  of  the  shoulder  and  chest  (s)  and  of  the 
thumb  and  fingers  (h)  have  been  evoked  by  strong  faradization.  Similarly 
the  shaded  area  marked  "EYES"  indicates  a  field  of  free  surface  of  cortex 
which  under  faradization  yields  conjugate  movements  of  the  eyeballs.  The 
conditions  of  obtainment  of  these  reactions  separates  them  from  those  char- 
acterizing the  "motor"  area.  (Sherrington.) 

it  has  been  painted  with  cocaine  or  after  the  administration  of 
chloral.  Moreover  the  latent  period  after  stimulating  the  gray 
matter  is  longer  (.065  second)  than  when  the  white  matter  is  di- 
rectly stimulated  (.045  second). 

376 


THE  NERVOUS  SYSTEM 


Characteristics  of  Movements  Excited  in  the  Cerebrum  —  By 

stimulation  of  the  cortex  coordinated  movements,  precisely  similar 
to  normal  voluntary  movements  are  elicited.  This  fact,  of  course, 
means  that  the  normal  tone  of  some  muscles  must  be  inhibited.  This 
inhibition  is  absent  during  strychnine  and  tetanus  poisoning  so  that 

Frontal 
association 


Parietal 

association 

area 


Temporo-occipital 
association  area 


Parietal 

.association 

area 


Frontal 

•association 

area 


Temporo-occipital 
association  area 


B 


Fig.  147. — Diagrams  suggesting  the  general  motor,  general  and  special  sensory 

and  the  association  areas  of  the  convex  and  mesial  surfaces 

of  the  cerebral  hemisphere.     (Morris.) 

under  the  influence  of  these  drugs  only  movements  are  obtained 
which  represent  those  of  the  stronger  set  of  muscles. 

The  part  played  by  inhibition  is  well  illustrated  by  the  eye 
movements.  When  the  convex  surface  of  the  inferior  frontal  con- 
volution on  the  right  side  is  stimulated  both  eyes  turn  toward  the 

378 


THE  NERVOUS  SYSTEM 

left.  This  movement  can  only  take  place  in  the  right  eye  by  a 
simultaneous  relaxation  of  the  right  external  rectus  and  contraction 
of  the  right  internal  rectus  and  the  reverse  of  these  events  in  the 
left  eye.  After  division  of  all  the  muscles  of  the  right  eye  except 
the  external  rectus  the  eye  will  be  constantly  turned  outward.  The 
same  stimulus  applied  now  will  cause  a  sufficient  relaxation  of  the 
right  external  rectus  to  permit  of  the  eye  returning  to  the  middle 
line. 

These  eye  movements  further  illustrate  the  bilateral  effect  of 
certain  unilateral  cortical  stimulation.  In  other  words  they  illus- 
trate that  every  movement  originating  in  the  cortex  is  a  purpose 
movement. 

The  Contrast  and  Interaction  between  the  Control  over  Move- 
ment Exerted  by  the  Cerebrum  and  Cerebellum  —  The  cerebellum 
also  plays  an  important  part  in  this  same  control.  Both  organs 
participate  in  the  maintenance  of  muscular  tone  and  both  are  able 
to  do  so  by  inhibition;  but  it  is  the  special  function  of  the  cere- 
bellum to  maintain  that  constant  tone  which  is  essential  to  attitude 
while  the  cerebrum  is  responsible  for  changing  activity. 

The  cerebellum  may  be  spoken  of  as  the  automatic  agent  of  the 
brain  in  the  influence  which  it  exerts  in  response  to  sensory  im- 
pulses, while  the  cerebrum  is  the  voluntary  agent;  the  cerebellum 
is  the  special  center  for  continuous  muscular  contraction,  while  the 
cerebrum  is  the  center  for  changing  movements  and  may  be  played 
upon  by  other  afferent  impulses  leading  to  voluntary  contraction  as 
well  as  by  impulses  through  the  proprioceptive  system.  The  cere- 
bellum may  be  viewed  as  a  special  receiving  organ  for  propriocep- 
tive impulses,  where  these  impulses  find  a  mechanism  capable  of 
passing  on  to  the  rest  of  the  central  nervous  system  impulses,  re- 
sulting in  equilibrium  and  normal  muscular  tone.  To  a  large 
degree  these  efferent  impulses  from  the  cerebellum  pass  through  the 
cerebrum  which  in  turn  uses  the  cerebellar  mechanism,  as  a  pre- 
pared, accurately  adjusted  and  sensitive  mechanism  for  the  produc- 
tion of  an  automatic  unconscious  coordination.  The  cerebrum  gives 
direction  to  this  coordination  in  the  voluntary  changes  of  activity 
for  which  it  alone  is  responsible. 

The  Difference  in  the  Functions  of  the  Cerebral  Motor  Areas  in 
Man  and  in  the  Animal  —  Effects  of  removal  of  the  motor  centers 
are  very  different  in  man  and  in  animals  even  so  high  in  the  scale 

380 


THE  NERVOUS  SYSTEM 

of  life  as  the  dog.  In  the  dog  the  first  effect  of  the  removal  of  the 
motor  area  is  a  very  severe  disturbance  of  the  dog's  power  of  move- 
ment. The  muscles  on  the  side  opposite  to  the  operation  are  much 
weaker.  Recovery  takes  place  after  a  few  weeks,  such  complete 
recovery  that  the  animal  can  be  taught  new  movements  involving 
the  use  of  the  affected  limb. 

In  the  monkey  recovery  is  less  complete.  There  is  some  perma- 
nent awkwardness  and  the  immediate  effect  is  one  of  absolute  par- 
alysis. 

In  man  lesions  of  the  motor  area  produce  still  more  serious 
effects.  There  is  absolute  paralysis  at  first  and  only  a  very  trivial 
amount  of  recovery,  if  the  pathological  condition  can  be  removed. 
The  amount  of  recovery  will  be  inversely  proportional  to  the 
amount  of  the  motor  area  destroyed  by  the  lesion  or  its  operative 
removal. 

These  graded  consequences  of  destruction  of  the  motor  area 
among  animals  and  man  are  another  illustration  of  the  shifting  of 
nervous  activities  as  we  ascend  the  scale  of  life  from  that  region 
where  they  are  necessitated  by  direct  paths  and  few  association 
tracts,  activities  that  may  be  characterized  by  the  word  fateful,  to  a 
region  where  they  are  conditioned  by  any  one  set  of  a  host  of  affer- 
ent impulses  reaching  the  regions  in  question  along  any  set  of 
numerous  association  tracts  which  all  together  make  consciousness 
possible. 

In  man  there  exists  the  possibility  of  a  greater  variation  in 
response,  or,  in  other  language,  a  greater  variety  of  movements. 
Actions  become  based  on  motive  and  new  cerebral  activities  impos- 
sible in  the  animal  are  learned. 

In  man  all  must  be  learned  at  the  expense  of  education.  Man 
comes  into  the  world  with  comparatively  few  laid  down  paths.  For 
many  years,  as  a  reactive  organism,  he  is  far  inferior  to  the  lower 
animals.  It  is,  however,  only  in  virtue  of  this  fact  that  in  him  a 
greater  adaptation  of  action  to  intelligent  needs  becomes  possible. 

The  Dependence  of  the  Motor  Area  upon  Different  Impulses  to 
it  —  In  speaking  of  the  motor  area  as  a  center  for  voluntary  im- 
pulses we  must  not  consider  that  the  whole  chain  of  events  leading 
to  a  movement  occurs  in  the  motor  area,  or  that  all  movements  arise 
there.  Like  the  cells  in  the  anterior  horns  of  the  spinal  cord  the 
cells  of  the  motor  area  are  utilized  as  the  last  chain  in  a  series  of 

382 


THE  NERVOUS  SYSTEM 

cerebral  events  and  are  played  upon  by  other  impulses  partici- 
pating in  the  complex  mechanism  which  alone  makes  possible  choice 
of  action. 

The  Receiving  End  of  the  Mechanism  —  Having  discussed  the 
motor  or  discharging  mechanism,  let  us  turn  to  the  other  end  of  the 
chain  of  cerebral  events,  the  receiving  mechanism.  Of  first  im- 
portance is  that  region  of  the  brain  which  is  most  closely  related 
to  the  perception  of  tactile  and  muscular  sensibility.  Many  facts 
indicate  that  the  ascending  parietal  convolution  is  the  seat  of 


AWDITOR.Y 


Fig.  148.  —  Outline  drawing  of  the  external  surface  of  the  hemisphere.    Shaded 

portion  represents  the  receptive  area  for  tactile, 

auditory  and  visual  sensations. 

the  direct  perception  of  tactile  and  muscular  sensibility.     (Figs. 
148  to  149.) 

(1)  Widespread  lesions  in  the  motor  area  will  not  only  produce 
paralysis  but  more  or  less  complete  hemi-anesthesia. 

(2)  Lesions  posterior  to  the  fissure  of  Rolando  including  the 
posterior  central  convolution,  the  superior  and  inferior  parietal, 
and  the  supramarginal  convolutions  are  characterized  by  more  pure 
disturbances  of  sensation. 

(3)  In  the  same  manner  certain  more  posterior  lesions  of  this 
and  the  motor  area  of  the  brain,  causing  Jacksonian  epilepsy,  may 
be  preceded  by  sensory  aura.    The  sensory  areas  are  less  definitely 
located  than  the  motor  areas  and  may  overlap  and  in  part  invade 

384 


THE  NERVOUS  SYSTEM 

the  motor  area.  The  sensory  perceptions  located  in  this  region  of 
the  brain  include  the  sense  of  pressure,  of  temperature  and  the 
muscular  sense  that  is  all  sensations  involved  in  stereognostic  per- 
ception. The  sense  of  pain  is  not  included  in  this  perception.  It 
includes  only  those  single  perceptions  which  are  needed  for  the 
perception  of  form,  size  and  solidity.  Lesions  in  region  mentioned 
cause  chiefly  a  disturbance  of  stereognostic  perception,  a  symptom 
named  astereognosis.  With  these  lesions  the  sense  of  pain  is  little 
if  at  all  affected.  Even  in  tabes  dorsalis  there  is  atrophy  of  the 
posterior  central  convolution. 


OLFACTORY     Afc.E.A. 

Fig.  149. — Inner  surface  of  the  same  hemisphere. 

The  impulses  of  these  sensations  ascend  in  the  mesial  fillet  to 
the  optic  thalamus  and  pass  thence  by  a  new  set  of  fibers  through 
the  hinder  limb  of  the  internal  capsule  to  the  parietal  region. 

The  thalamus,  however,  sends  fibers  to  other  portions  of  the 
brain.  Cortical  lesions  of  the  central  convolutions  never  produce 
complete  hemianesthesia,  so  that  while  the  posterior  central  or 
ascending  parietal  convolution  is  the  chief  cerebral  receiving  station 
for  tactile  and  muscular  sensations,  widely  separated  other  portions 
of  the  brain  may  participate  in  this  function. 

Visual  Perception  —  This  is  located  in  the  occipital  lobe,  in  the 
cuneus  and  convolutions  bordering  the  calcarine  fissure.  Very  defi- 
nite evidence  exists  in  support  of  this  fact. 

386 


THE  NERVOUS  SYSTEM 

(1)  Excision  of  one  occipital  lobe  causes  crossed  hemianopsia, 
i.e.,  blindness  in  the  half  of  each  retina  which  is  opposite  to  that 
of  the  extirpated  lobe.     This  bilateral  effect  is  explained  by  the 
manner  in  which  the  optic  fibers  cross  in  the  optic  chiasma. 

(2)  Stimulation  of  the  occipital  lobe  in  an  animal  causes  the 
eyes  to  move  toward  the  opposite  side  because  of  a  revival  of  past 
visual  sensations. 

(3)  The  eyes  will  move  downward  and  to  the  opposite  side  if 
the  upper  part  of  the  occipital  lobe  is  stimulated  and  upward  and 
to  the  opposite  side  if  the  lower  portion  of  occipital  lobe  is  stim- 
ulated. 

(4)  From  the  hinder  end  of  the  pulvinar  and  external  genicu- 
late  body  which  receive  the  optic  nerves,  fibers  arise  which  pass 
through  the  hinder  end  of  the  internal  capsule  and,  as  the  optic 
radiations,  to  occipital  lobes. 

(5)  Pathological  lesions  fully  confirm  these  conclusions. 
Perception  of  hearing  —  Situated  in  the  superior  temporal  con- 
volution; but  probably  not  entirely  here. 

(1)  Extirpation  of  the  superior  temporal  convolution  in  mon- 
keys produces  marked  disturbances,  but  not  a  complete  disturbance 
of  hearing. 

(2)  Cortical  lesions  of  the  superior  temporal  convolution  in  man 
produce  varying  degrees  of  deafness. 

(3)  Stimulation   of  the   superior   temporal   convolutions  will 
cause  animals  to  prick  up  their  ears  as  if  sounds  were  heard. 

(4)  From  the  auditory  nucleus  in  the  medulla  nerve  fibers  pass 
to  the  trapezium,  and  thence  by  the  lateral  fillet  to  the  inferior  cor- 
pora quadrigemina.     From  this  body  and  the  internal  geniculate 
body  they  pass  into  the  hinder  parts  of  the  internal  capsule  and 
thence  as  the  auditory  radiations  to  the  superior  temporal  convolu- 
tions.   The  fibers  from  the  two  internal  geniculate  bodies  decussate 
across  the  middle  line  in  Guddens'  commissure  which  form  the  pos- 
terior fibers  of  the  optic  chiasma. 

Smell  and  Taste  Perception  is  located  in  the  hippocampal  gyrus, 
the  dentate  convolution  and  in  that  portion  of  the  limbic  lobe  known 
as  the  gyrus  fornicatus  which  immediately  borders  the  superior 
surface  of  the  corpus  callosum.  Among  animals  the  sense  of  smell 
is  a  far  more  important  sense  than  in  man.  Its  connections  are, 
therefore,  widespread.  In  man  it  is  only  natural  to  expect  that  the 

388 


THE  NERVOUS  SYSTEM 


same  widespread  connections  should  exist  and,  perhaps,  be  all  the 
less  well  defined  on  account  of  the  contemporaneous  huge  develop- 
ment of  the  rest  of  the  brain  and  the  corresponding  disappearance 
of  the  acuteness  of  the  perception  of  smell  (see  Fig.  147). 

Electrical    stimulation    of    the    hippocampal    convolution    has 
caused  movements  of  the  lips  and  nostrils.    Ablation  experiments 

Broca  area  third  inf.  frontal 
,       co-ordination  of  speech  muscles. 

Ascending  parietal  convol.  motor 
/       area  for  hand  graphic  images. 

Ascending  parietal 

r     motor  area  for 

'        mouth  and  larynx. 

Supramarginal 
convol. 
auditory 
word  images. 


Visual 
area 
cuneus. 


Sup.  temp,  convol.  ' 
auditory  area. 


Mid.    temp,   convolution 
word     understanding. 


Fig.    150. — Convex   surface    of   left    cerebral    hemisphere    and    diagrammatic 
presentation  of  the  areas  suggested  as  concerned  with  speech.     (Morris.) 

have  not  given  much  information.  The  most  valuable  information 
is  to  be  derived  from  the  connections  in  the  lower  animals.  In  addi- 
tion to  the  portions  of  the  brain  which  we  have  mentioned,  the  pos- 
terior part  of  the  inferior  surface  of  the  frontal  lobe  and  the  olfac- 
tory lobe  and  the  anterior  commissure  must  be  included. 

Association  Areas  and  the  Significance  of  Association  of  Cere- 
bral Impulses  and  Their  Relation  to  Thought  and  Speech  —  The 
areas  of  the  brain  which  we  have  identified  with  perception  and 
action  occupy  a  comparatively  small  amount  of  the  cortex  of  the 
brain. 

390 


THE  NERVOUS  SYSTEM 

Inasmuch  as  the  cerebral  processes  transpiring  within  these 
areas  cannot  be  unraveled,  they  have  been  termed  the  silent  areas, 
and  as  the  living  being  rises  in  the  scale  of  intelligence  these  areas 
become  relatively  larger.  They  make  up  by  far  the  larger  portion 
of  man's  brain. 

When  we  attempt  to  analyze  the  cerebral  processes  accompany- 
ing a  single  combination  of  sensations  and  the  infinite  variety  of 
cerebral  processes  representing  the  result  of  the  influence  of  these 
sensations  collectively,  it  is  quite  evident  that  even  simple  forms  of 
cerebral  activity  are  very  intricate.  Thinking  is  only  possible 
because  of  man 's  power  to  quickly  call  into  use,  or  in  other  words 
to  associate,  many  portions  of  the  brain  which  have  to  do  with  pre- 
vious sensations.  So  intricate  does  this  activity  become  that  a  large 
part  of  the  advantage  of  the  means  for  this  association  becomes  lost 
without  provision  for  cerebral  short  cuts.  The  association  itself  is 
primarily  accomplished  by  connecting  neurons  and  the  process  of 
association  of  impulses  or  a  set  of  impulses  which  have  been  linked 
together  as  a  unit  (such  a  unit  often  constituting  a  concept  or  idea) 
is  facilitated  by  the  laying  down  of  other  fibers  or  even  tracts  which 
furnish  short  cuts  and  which  make  possible  the  more  rapid  revival 
of  not  only  past  perceptions  as  they  happen  to  be  related  to  a  par- 
ticular stimulus  starting  the  cerebral  activity,  but  also  whole  groups 
of  perceptions,  taken  as  a  whole. 

The  Grouping  of  Perception  Made  Possible  by  Speech  —  These 
short  cuts,  therefore,  make  possible  education  and  memory. 

Speech  —  In  the  development  of  man  the  rapid  association  of 
groups  of  impulses  constituting  concepts  has  been  greatly  facili- 
tated by  the  adoption  of  audible  symbols  for  concepts.  By  this 
invention  man  has  rendered  possible,  as  a  result  of  his  greater  power 
of  association  and  his  power  of  phonation,  an  almost  indefinite 
enlargement  of  the  power  of  reviving  instantaneously  past  associa- 
tions of  great  complexity.  Upon  this  invention  alone  depends  our 
power  of  intricate  thinking. 

The  Varieties  of  Aphasia  —  Various  disturbances  of  the  power 
of  speech  demonstrate  more  closely  the  cerebral  processes  upon 
which  it  is  based.  A  number  of  different  forms  of  aphasia  have 
been  described. 

(1)  Motor  Aphasia — This  form  of  aphasia  has  been  described 
as  an  inability  to  speak  though  the  individual  understands  every- 

392 


THE  NERVOUS  SYSTEM 

thing  which  is  said  to  him  and  suffers  no  impairment  of  his  intelli- 
gence. This  form  of  aphasia  has  been  for  a  long  time  associated 
with  a  lesion  in  the  third  left  frontal  convolution  immediately  an- 
terior to  the  lower  end  of  the  ascending  frontal  convolution.  This 
area  has  been  called  Broca  's  area,  after  the  man  who  first  described 
the  aphasia  and  its  associated  lesion.  The  traditional  association  of 
motor  aphasia  with  a  lesion  in  the  third  left  (right-handed  people) 
convolution  has  been  so  strong  that  few  clinicians  do  not  accept  it 
outright.  Nevertheless  the  association  will  not  bear  investigation. 
Theoretically  it  should  not.  The  complex  character  of  all  the  asso- 
ciations necessary  to  speech  cannot  be  grouped  in  one  center  of  the 
brain,  and  the  same  argument  contradicts  with  equal  force  the  too 
strict  localization  of  sensory  aphasia  with  the  area  of  Wernicke. 

Undoubtedly  near  Broca 's  area  in  the  cortex  are  the  motor  cen- 
ters for  the  muscles  of  the  larynx,  but  the  majority  of  cases  of  motor 
aphasia  are  really  a  species  of  anarthria,  and  upon  autopsy  are' 
found  to  be  associated  with  lesions  in  other  locations  particularly 
in  the  external  capsule  and  the  anterior  portion  of  the  internal 
capsule.  No  good  ground  exists  for  distinguishing  between  motor 
aphasia,  when  intelligence  is  unimpaired,  and  the  type  of  aphasia 
described  below  by  the  word  anarthria.  The  majority  of  cases  de- 
scribed as  motor  aphasia  are  associated  with  impaired  intelligence 
and  belong  in  the  second  variety  of  aphasia. 

(2)  Sensory  Aphasia  or  Aphasia  of  Wernicke  —  This  form  of 
aphasia  is  associated  with  lesions  in  the  supra-marginal  and  angular 
gyri  and  posterior  end  of  the  second  temporal  convolutions.    In  this 
condition  there  may  be  limited  power  of  speech,  but  there  is  impair- 
ment of  intelligence  and  especially  of  the  appreciation  of  spoken 
words.    There  may  also  be  loss  of  power  to  recognize  written  words 
(alexia).    The  motor  portion  of  this  aphasia  is  due  rather  to  the 
individual's  inability  to  understand  his  own  spoken  words. 

(3)  Anarthria  —  In  this  condition  there  is  a  pure  impairment 
of  the  motor  powers  of  expression.    It  is  generally  associated  with 
a  lesion  in  the  external  capsule.    Appreciation  of  speech  written 
and  spoken  is  perfect  and  intelligence  is  unaltered. 

Wernicke 's  area  must  be  regarded  as  only  one  of  the  great  asso- 
ciation centers  of  the  brain  between  various  forms  of  perception  and 
between  them  and  motion.  Lesions  in  them  mean  a  blunting  of 
intelligence,  because  the  power  of  forming  complete  concepts  is 

394 


THE  NERVOUS  SYSTEM 

lacking  though  the  individual  may  be  in  perfect  possession  of  the 
logical  faculty.  In  true  insanity  there  is  an  impairment  of  the 
higher  association  centers  located  in  the  prefrontal  region.  The 
simpler  concepts  are  perfectly  formed  but  the  power  of  grouping 
these  in  a  manner  necessary  for  the  processes  involved  in  logical 
thought  is  lost.  By  means  of  the  myelinization  method  Flechsig  has 
been  able  to  divide  up  the  cerebral  cortex  info  some  36  areas.  Eight 
of  these  belong  to  the  regions  which  have  been  described  as  asso- 
ciated with  the  action  end  or  primary  projection  areas  of  the  cor- 
tex. 

In  the  case  of  seven  areas  the  function  is  uncertain.  The  areas 
do  not  possess  either  projection  fibers  or  apparently  association 
fibers. 

Eighteen  areas  are  provided  with  short  association  fibers.  They 
may  be  termed  intermediate  areas. 

Three  areas  possess  long  association  fibers.  They  may  be  termed 
the  large  and  important  association  areas.  One  of  these  occupies 
the  prefrontal  region  on  both  the  internal  and  external  surface  of 
the  cortex.  A  second  occupies  the  2nd  and  3rd  temporal  convolu- 
tions and  the  third  a  large  area  on  the  external  surface  of  the  cor- 
tex, including  the  posterior  portion  of  the  supramarginal  convolu- 
tion and  extending  posteriorly  to  the  visual  perception  area  in  the 
cuneus.  Until  comparatively  recently  the  nuclei  of  gray  matter 
grouped  under  the  name  of  the  corpus  striatum  and  including  the 
lenticular  nucleus  and  the  caudate  nucleus  were  regarded  as  simi- 
lar in  function  to  the  optic  thalamus  and  like  it  to  constitute  merely 
relay  stations  for  impulses  on  the  way  to  and  from  the  brain. 
After  destroying  these  nuclei,  however,  degenerated  fibers  are 
found  passing  from  them  to  the  optic  thalamus.  These  nuclei, 
therefore,  send  out  efferent  fibers  to  lower  cerebral  nuclei.  They 
are  also  known  to  receive  fibers  from  the  optic  thalamus  and  the 
olfactory  tracts.  Such  connections  indicate  that  these  nuclei 
are  independent  masses  of  gray  matter  capable  of  receiving  afferent 
impulses  from  below  and  of  sending  out  independent  efferent  im- 
pulses. They  must  be  regarded  as  relay  stations  within  the  brain 
itself  between  the  cortex  and  the  lower  thalamic  centers. 

In  a  series  of  animals  representing  an  ascending  scale  of  cere- 
bral development  the  corpus  striatum  occupies  a  relatively  less 
importance  in  cerebral  activities.  In  birds,  on  the  other  hand,  they 

396 


THE  NERVOUS  SYSTEM 

have  their  greatest  development.  It  would  appear  that  they  repre- 
sent then  a  divergent  development  in  birds,  taking  over  an  in- 
creasing number  of  functions  in  them,  while  in  mammals  they  are 
retrogressive,  their  functions  being  shifted  to  the  pallidium  or 
cerebral  hemispheres.  Stimulation  of  these  nuclei  produces  no 


Genu  of  corpus  callosum. 
Anterior     horn     of     lateral 
ventricle. 

Caudate  nucleus. 
Anterior     limb     of     internal 
capsule. 

Cavum    septi    pellucidi. 

Genu  of  internal   capsule. 

Column  of  fornix. 

Globus   pallidus    (of  nucleus 

lentiformis). 
Fasciculus       mammlllothala- 

micus. 

Posterior    limb    of    Internal 
capsule. 

Thalamus. 

Retrolenticular    part    of    In- 
ternal capsule. 

Hippocampus. 

Splenium. 

Chorioid   plexus. 


Gyrus  cingull. 
Calcarine   sulcus. 


Lentlculo-caudate  fibres. 

U; Claustrum. 

-Insula. 

^_  Putamen     nucleus     lentifor- 
mis. 

Internal    capsule    with    ansa 
lenticularis   fibres    in  blue. 


Tail   of  caudate  nucleus. 

Optic    radiation. 
Tapetum. 

Optic  radiation  passing  back 
to  white  line  in  the  area 
striata. 


Fig.  151. — Horizontal  section  through  the  right  cerebral  hemisphere  at  the 
level  of  the  widest  part  of  the  lentiform  nucleus.     (Cunningham.) 


movements.  In  the  monkey  their  destruction  is  followed  by  no 
definite  results.  In  man  lesions  in  these  bodies  produce  tremors 
in  the  execution  of  willed  movements  and  an  increased  tonicity  of 
the  muscles,  functions  resembling  those  of  the  cerebellum. 

Experimental  evidence  of  the  nature  of  the  application  of  iso- 
lated heat  and  cold  to  the  anterior  part  of  the  corpus  striatum  indi- 
cates that  this  portion  of  gray  matter  contains  the  chief  thermo- 

398 


THE  NERVOUS  SYSTEM 


* 


taxic  center  of  the  body.  Cooling  it,  for  instance,  produces  shiver- 
ing and  increased  heat  production  in  the  body,  while  warming  it 
produces  the  opposite  effect. 

The  Histological  Structure  of  the  Cortex  —  The  preceding  lo- 
calization of  nervous  function  within  the  cerebral  cortex  is  largely 
confirmed  by  a  study  of  the  histological  structure  of  the  cortex.  The 
cortex  consists  of  many  layers  of  cells  imbedded  in  a  neuroglia  sup- 
porting framework.  As  the 
Purkinje  cells  are  characteris- 
tic of  the  cerebellum,  so  the 
pyramidal  cell  belongs  pecu- 
liarly to  the  cerebral  cortex. 
It  is  a  cone-shaped  or 
pear-shaped  cell  with  one 
large  apical  dendrite  which 
runs  towards  the  surface  to 
break  up  in  the  most  super- 
ficial layers  of  the  cortex  into 
number  of  branches.  Den- 
drites  are  given  off  from  the 
sides  of  "the  cell.  The  axon 
starts  in  the  base  of  the  cell 
and  passes  down  into  the 
white  matter,  giving  off  col- 
laterals in  its  course. 

Some  fibers  reach  the  cor- 
pus callosum,  others  the  in- 
ternal capsule,  and  others  ad- 
jacent parts  of  the  cortex. 

There  may  be  distin- 
guished four  or  five  layers  of 
cells  within  the  cortex.  (Fig. 
152.)  (1)  Outer  fiber  lamina 
or  molecular  layer  contains 


Fig.  152. — Cerebral  cortex,  diagrammatic 
section. 


On  the  left,  the  cellular  layers;  on  the     f  ^   spindle-shaped   the 

rnr      circToma     /-*T     riKTocj"     nn    rno    ovry^mo  *  A 


right,  systems  of  fibres;  on  the  extreme 
left  a  sensory  fibre  is  seen  ascending; 
1,  2,  3,  4  the  four  layers  of  cells;  2  and  3 
representing  pyramidal  cells  of  differing 
size. 


400 


processes  of  which  run  paral- 

lel  to  the  surface.     The  layer 

. 

is    mostly    composed    of   the 

branching   dendrites    of   the 


THE  NERVOUS  SYSTEM 

cells  of  the  deeper  layers.  (2)  Outer  cell  lamina  or  pyramidal 
cell  layer. 

It  contains  three  varieties  of  pyramidal  cells  arranged  from 
without  inwards  into  (a)  small  pyramidal  cells,  (b)  medium  pyra- 
midal cells,  (c)  large  pyramidal  cells.  (3)  Stellate  cell  layer  or 
middle  cell  lamina,  as  indicated,  contains  stellate-shaped  cells.  (4) 
Inner  fiber  lamina,  composed  of  many  nerve  fibers  and  in  certain 
portions  of  the  brain,  particularly  the  motor  areas,  this  layer  con- 
tains large  solitary  cells,  the  cells  of  Betz.  (5)  The  polymorphous 
cell  layer  and  inner  cell  lamina,  containing  cells  of  many  types,  but 
among  which  the  pyramidal  cells  predominate.  Some  of  the  pyra- 
midal cells  are  inverted,  so  to  speak,  their  axons  run  to  the  surface. 
These  are  called  cells  of  Marinotti.  Other  cells,  Golgi  cells,  possess 
freely-branching  axons  ending  near  the  cell. 

The  fibers  from  the  white  matter  of  the  brain  run  toward  the 
surface,  giving  off  a  rich  meshwork  of  fibers  to  the  various  layers 
of  gray  matter.  Other  fibers  run  parallel  to  the  surface  and  on  the 
very  surface  of  the  brain.  These  fibers  in  some  regions,  especially 
the  hippocampal  region,  are  so  well  marked  that  they  are  termed 
the  tangential  fibers. 

Another  layer  of  tangential  fibers  is  found  between  the  molecu- 
lar layer  and  the  pyramidal  cell  layer.  It  is  called  the  outer  line  of 
Baillance. 

Internal  to  the  granular  layer  is  another  layer  of  tangential 
fibers,  the  inner  line  of  Baillance. 

In  the  occipital  region  there  is  a  special  tangential  layer  running 
through  the  middle  of  the  granular  layer.  It  is  called  the  line  of 
Gennari. 

Identification  of  Function  "by  Means  of  Histological  Detail  — 
The  thickness  of  these  various  layers  furnish  information  as  to  the 
function  of  the  various  portions  of  the  cerebral  cortex. 

In  the  ascending  frontal  convolution  the  cells  of  Betz  are  numer- 
ous and  larger  than  in  any  other  region.  The  pyramidal  cell  layer 
is  also  very  thick. 

In  the  visuo-sensory  area  the  stellate  cell  layer  or  granular  layer 
is  thickest  and  the  line  of  Gennari  present. 

In  association  areas,  the  parietal,  temporal  and  frontal  the  outer 
cell  layer  or  pyramidal  cell  layer  is  very  thick.  It  is  the  most 

402 


THE  NERVOUS  SYSTEM 

marked  feature  of  sections  in  these  regions.  These  cells,  therefore, 
have  to  do  with  the  higher  functions  of  association. 

In  animals  lower  than  man,  the  ape  and  dog,  less  of  the  brain  is 
occupied  with  areas  possessing  the  histological  structure  identified 
with  association.  In  still  lower  animals,  the  rabbit,  the  polymor- 
phous layer  is  three  times  the  thickness  of  the  pyramidal  layer. 

We  may,  therefore,  assign  to  the  cells  of  Betz  motor  function,  to 
the  pyramidal  cells  associative  functions,  and  to  the  polymorphous 
cells  functions  concerned  in  the  getting  of  food  and  the  gratification 
of  the  various  sensuous  instincts. 

When  the  cerebral  activities  are  deficient  either  because  of  dis- 
ease or  congenital  defects,  the  cells  are  less  numerous  in  the  regions 
controlling  the  deficient  functions. 

Time  of  Certain  Cerebral  Activities  —  The  time  of  the  various 
reactions  in  which  the  brain  is  concerned  is  of  interest.  They  may 
be  recorded  by  an  electrical  apparatus  which  marks  the  moment  of 
the  application  of  any  stimulus  and,  through  a  shunt  circuit,  the 
voluntary  reaction  of  the  patient. 

The  time  for  the  reaction  to  sight  stimuli  is  .186  to  .222  of  a 
second ;  to  hearing  .115  to  .182  second ;  to  electrical  stimulation  of 
skin  .117  to  .201  second. 

The  time  may  be  lengthened  .006  second  by  fatigue  of  the  reac- 
tion, or  by  a  dilemma,  involving  choice  by  the  individual. 

It  may  be  shortened  by  practice,  or  by  increase  in  strength  of 
the  stimulus. 


THE   SYMPATHETIC   NERVOUS  SYSTEM 

The  nerves  passing  from  the  central  nervous  system  to  the  vari- 
ous portions  of  the  body  may  be  divided  into  two  different  classes. 
First  those  conveying  motor  impulses  from  the  spinal  cord  and 
brain  and  those  returning  sensory  impulses.  In  addition  to  these 
nerves  there  is  another  class  of  nerves  issuing  with  the  cranial 
nerves  and  the  anterior  and  posterior  roots  of  the  spinal  nerves 
which  convey  afferent  impulses  from  and  efferent  impulses  to  the 
blood  vessels  and  viscera. 

Briefly,  they  supply  smooth  muscle  and  glandular  tissue. 

The  nerves  of  this  second  class  are  connected  with  peripheral 

404 


THE  NERVOUS  SYSTEM 

ganglia  and  differ  histologically  from  other  nerves.  All  these  facts 
warrant  their  classification  as  a  separate  system. 

It  is  called  the  vegetative  nervous  system,  and  may  be  divided 
into  the  autonomic  or  cranial  portion  of  the  vegetative  nervous 
system  and  the  spinal  portion  or  the  sympathetic  nervous  system. 

In  contradistinction  from  it  we  may  call  the  other  nerves  of  the 
body  those  innervating  skeletal  muscle  and  returning  sensory  im- 
pulses the  somatic  nervous  system. 

The  vegetative  nerves  of  the  third  cranial  nerve  pass  with  the 
third  nerve  to  the  orbit.  Leaving  the  branch  of  the  third  nerve 
which  supplies  the  inferior  oblique  muscle,  they  enter  the  lenticular 
ganglion.  From  this  ganglion  their  axons  are  continued,  after  inter- 
ruption, as  the  short  ciliary  nerves  to  the  sphincter  pupili  muscle 
and  the  ciliary  muscles. 

The  vegetative  nerves  of  the  7th  cranial  nerve  are  contained  in 
the  nerve  of  Wrisberg.  This  nerve  also  contains  fibers  of  taste  from 
the  tongue.  The  fibers  belonging  to  the  vegetative  system,  however, 
leave  the  7th  nerve  as  the  chorda  tympani  and  later  join  the  lingual 
nerve  and  with  this  pass  to  the  submaxillary  ganglion.  From  this 
ganglion  it  supplies  dilator  fibers  and  secretory  fibers  to  the  sub- 
maxillary  and  sublingual  salivary  glands.  The  chorda  tympani 
also  sends  fibers  to  the  sphenopalatine  ganglion  from  which  post- 
ganglionic  fibers  supply  the  mucous  membrane  of  the  nose  and  soft 
palate  and  upper  part  of  the  pharynx. 

The  vegetative  fibers  of  the  9th  nerve  pass  to  the  otic  ganglion. 
From  this  ganglion  its  post  ganglionic  fibers  pass  to  the  parotid 
gland  and  supply  it  with  vaso-dilator  and  secretory  fibers. 

Practically  all  of  the  vagus  nerve  may  be  regarded  as  belonging 
to  the  visceral  system.  The  jugular  ganglion  represents  its  ganglion 
cell  station.  The  ganglion  of  the  trunk  of  the  vagus  probably  cor- 
responds to  a  posterior  spinal  ganglion  and  is  connected  with  affer- 
ent nerves  of  the  vagus  nerve  only.  As  has  already  been  mentioned, 
it  supplies  motor  fibers  to  the  alimentary  tract  as  far  as  the  ileocolic 
sphincter,  inhibitory  fibers  to  the  heart,  motor  fibers  to  the  bronchi 
and  secretory  fibers  to  the  stomach  and  pancreas. 

Sympathetic  Fibers  of  the  Spinal  Nerves  —  Each  spinal  nerve 
gives  off  fibers  which  participate  in  the  formation  of  the  visceral 
system.  They  are  represented  in  the  anterior  nerve  roots  by  the 
small  medullated  fibers.  (Fig.  153.)  These  leave  the  anterior  divi- 

406 


THE  NERVOUS  SYSTEM 


Fig.  153. — Sections  across  parts  of  the  roots  of  various  nerves  of  the  dog,  to 

show  the  variations  in  size  of  their  constituent  fibres.     (Quain.) 
(The  nerves  were  stained  with  osmic  acid,  and  the  sections  are  all  drawn 
to  one  scale.)  , 

A,  from  one  of  the  upper  roots  of  the  accessory. 

B,  a  rootlet  of  the  hypoglossal. 

C,  from  the  first  cervical  ventral  root. 

D,  from  the  second  thoracic  ventral  root. 

sion  of  the  spinal  nerves  and  run  to  one  set  of  ganglia  but  ter- 
minate in  one  of  two  sets  of  ganglia.  One  of  these  sets  of  ganglia 
forms  a  chain  of  ganglia  lying  close  to  the  vertebral  column.  In 
general  there  may  be  said  to  be  one  ganglion  for  each  vertebral 
segment  of  the  column  in  the  thoracic  and  lumbar  region  and  three 
ganglia  for  the  cervical  region. 

The  second  series  of  ganglia  are  the  cardiac  plexus  at  the  root 

408 


THE  NERVOUS  SYSTEM 


of  the  lung  and  base  of  the  heart,  the  solar  plexus  around  the  celiac 
axis,  the  superior  and  inferior  mesenteric  plexuses  around  the  origin 
of  the  superior  and  inferior  mesenteric  arteries,  the  hypogastric 
and  pelvic  plexuses,  in  front  of  the  body  of  the  5th  lumbar  vertebra. 
We  may  call  the  spinal  ganglia  the  lateral  series  of  ganglia  and 
the  ganglia  in  the  large  plexuses  around  the  great  vessels  the  col- 
lateral ganglia.  Another  set  of  plexuses  more  distal  still,  exists  in 
the  walls  of  the  intestines.  They  are  the  plexuses  of  Meissner  and 
Auerbach.  Though  called  terminal  ganglia  they  contain  no  gan- 


Spinal  ganglion 


Spinal 
cord 


Afferent  fibre 
}  Efferent  fibres 


Sympathetic 
efferent  fibres 
Sympathetic  ganglion-flc* 


i 


Sympathetic  afferent  fibres 


Fig.  154. — Plan  of  construction  of  a  typical  spinal  nerve.     (Quain.) 

glion  cells  and  are  rather  to  be  viewed  as  sites  of  interlacing  of 
nerve  fibers  which  suffer  no  interruption  in  passing  through  them. 
All  of  the  sympathetic  nerves  leaving  the  anterior  division  of  the 
spinal  nerves  pass  to  the  spinal  or  lateral  ganglia.  As  they  are 
medullated  they  are  called  white  rami  communicantes.  Some  of 
them  end  in  a  terminal  arborization  around  the  cells  of  these  gan- 
glia ;  others  pass  through  these  ganglia  without  interruption  to  end 
around  cells  in  the  collateral  series  of  ganglia.  All  these  nerve 
fibers  are  called  preganglionic  nerve  fibers.  From  the  cells  around 
which  these  preganglionic  fibers  end  axons  are  given  off  which  are 
non-medullated  and  are  called  post-ganglionic  fibers.  (Figs.  154 
and  155.) 

No  sympathetic  nerve  has  more  than  one  of  these  :  iterruptions 

410 


THE  NERVOUS  SYSTEM 


between  its  origin  and  destination.  Many  of  the  axons  of  the  cells 
in  the  spinal  ganglion  run  back  from  the  ganglion  to  an  anterior 
spinal  nerve,  of  a  different  level,  bend  around  again  to  be  distrib- 
uted with  the  fibers  of  such  an  anterior  or  posterior  spinal  nerve. 

As  they  pass,  therefore,  between  the  ganglia  and  the  spinal 
nerves  they  are  also  called  rami  communicantes,  and  because  they 
are  not  medullated  they  are  called  gray  rami  communicantes. 


|  Cells  of 
lat.  horn 


fost-ganglionic , 
in  spinal  nerve 


Spinal 
cord 


Pre-ganglionic  fvbre 
in  sympathetic  »*"><> 


Distal  y—. 
in  sympafr 


fost-ganglionic  fibre. 
in;  sympathetic 


Post-ganqlionic  fil>re_ 
in  spinal/  nerve 


Fibre  in  sympathetic  cord 
gassing  through  two  ganglia 

Fig.  155. — Diagram  of  sympathetic.    (Quain.) 

Each  gray  ramus  communicans  is  distributed  to  only  an  area 
of  the  body  which  corresponds  to  the  level  at  which  it  is  given  off. 

A  white  ramus,  on  the  other  hand,  may  run  a  long  distance 
before  it  terminates  around  a  ganglionic  cell  from  which  its  post- 
ganglionic  fiber  is  given  off.  Stimulation  of  one  white  ramus  will 
cause  impulses  in  several  gray  rami. 

412 


THE  NERVOUS  SYSTEM 

The  spinal  ganglia  of  the  upper  three  cervical  nerves  pass  to 
the  superior  cervical  sympathetic  ganglion.  Its  branches  of  distri- 
bution are  to  plexuses  around  the  carotid  arteries  and  their 
branches. 

It  sends  branches  to  the  tympanum  and  to  the  Vidian  nerve  and 
to  the  Gasserian  ganglion.  Many  fibers  reach  the  superior  cervical 
ganglion  from  the  first  five  dorsal  nerves.  These  fibers  reach  it 
after  first  passing  through  the  dorsal  spinal  ganglia.  They  repre- 
sent some  of  the  white  rami  which  have  long  preganglionic  fibers, 
for  their  ganglionic  cells  are  in  the  superior  cervical  ganglion. 
They  convey  the  following  impulses: 

1.  Vaso-constrictor  impulses  to  blood  vessels, 

2.  Dilator  impulses  to  the  pupil, 

3.  Secretory   (trophic?)   impulses  to  the  salivary  and  sweat 
glands, 

4.  Vaso-dilator  fibers  to  the  lower  lip  and  pharynx. 

The  same  five  dorsal  nerves  send  fibers  to  the  stellate  ganglion, 
a  large  ganglion  beneath  the  origin  of  the  subclavian  artery.  It 
communicates  by  two  cords  which  surround  the  subclavian  artery 
with  the  inferior  cervical  ganglion  of  the  sympathetic.  The  ring 
around  the  subclavian  is  called  the  ansa  Vienssens.  The  inferior 
cervical  ganglion  of  the  sympathetic  is  placed  between  the  superior 
and  middle  cervical  ganglion  above,  with  which  it  is  also  connected 
by  two  cords,  and  the  stellate  ganglion  below. 

From  the  cell  stations  of  these  fibers  in  the  stellate  ganglion 
post- ganglionic  fibers  of  the  upper  dorsal  nerves  are  given  off  to 
the  heart. 

They  convey  accelerator  and  augment  or  impulses  to  the  heart. 

Each  spinal  ganglion  is  not  only  connected  with  the  anterior 
spinal  nerve  by  a  gray  and  a  white  ramus  but  also  with  the  ganglia 
above  arid  below  it  by  two  connecting  cords. 

The  upper  limbs  are  supplied  by  nerves  coming  from  the  4th  to 
the  llth  dorsal  ganglia. 

They  convey : 

1.  Vaso-constrictor  impulses  to  the  blood  vessels  of  the  limbs, 

2.  Secretory  fibers  to  the  sweat  glands. 

The  lower  limbs  are  supplied  by  branches  of  the  llth  dorsal  to 
the  third  lumbar  ganglion. 
They  convey : 

414 


THE  NERVOUS  SYSTEM 

1.  Vaso-constrictor  impulses  to  the  vessels  of  the  lower  limb, 

2.  Secretory  impulses  to  the  sweat  glands  of  the  lower  limb. 
From  the  lower  6  dorsal  and  upper  3  to  4  lumbar  ganglia  fibers 

pass  to  the  abdominal  viscera. 
They  convey : 

1.  Vaso-constrictor  fibers  to  the  vessels  of  the  stomach  and 
small  intestines,  the  kidney  and  spleen, 

2.  Probably  vaso-dilator  fibers  as  well, 

3.  Muscular  inhibitory  impulses  to  the  stomach  and  small  in- 
testines, 

4.  Motor  fibers  for  the  ileocolic  sphincter. 

Nerves  from  the  lower  dorsal  and  upper  3  to  4  lumbar  nerves 
pass  to  the  pelvic  plexus  in  two  strong  cords  running  as  the  hypo- 
gastric  nerves  from  the  inferior  mesenteric  plexus  to  the  pelvic 
plexus. 

They  convey : 

1.  Vaso-constrictor  impulses  to  the  vessels  of  the  viscera, 

2.  Inhibitory  impulses  to  the  colon, 

3.  Both  motor  and  inhibitory  impulses  to  the  bladder, 

4.  Motor  fibers  to  the  retractor  penis, 

5.  Motor  fibers  to  the  uterus  and  vagina. 

Besides  the  autonomic  fibers  passing  in  the  hypogastric  nerves 
from  the  inferior  mesenteric  plexus  to  the  pelvic  plexus,  the  an- 
terior branches  of  the  second  to  the  fourth  sacral  nerves  furnish 
branches  of  autonomic  fibers  which,  without  making  connections 
with  any  lateral  ganglia,  unite  to  form  on  each  side  the  nervus 
Erigens.  This  nerve  passes  directly  to  the  pelvic  plexus  in  which 
its  fibers  suffer  interruption. 

They  convey : 

1.  Motor  impulses  to  the  bladder,  descending  colon  and  rectum, 

2.  Vaso-motor  impulses  to  the  vessels  of  the  pelvic  viscera, 

3.  Inhibitory  fibers  to  the  sphincter  of  the  bladder, 

4.  Dilator  fibers  to  the  vessels  of  the  penis  and  inhibitory  fibers 
to  the  retractor  penis. 


416 


QUESTIONS  AND  ANSWERS 


Pages  4-8 

Q.  What  is  the  function  which  the  nervous  system  has  been  developed  to 
perform? 

A.  To  make  possible  the  rapid  transmission  between  distant  portions  of 
the  body  of  changes  in  the  environment  of  groups  of  cells. 

Q.  What  important  stages  in  the  development  of  a  central  nervous  system 
are  represented  by  the  nervous  systems  of  invertebrates? 

A.  The  peripherally  placed  nervous  system  of  a  hydra  in  which  there  is 
but  slight  difference  between  the  protective  surface  epithelial  cell  and 
the  specialized  sensitive  and  conductive  epithelial  cell,  and  in  which  the  sensi- 
tive cell,  the  conductive  portion  and  contractile  tissue  constitute  one  cell. 
(Page  4.) 

The  peripherally  placed  nervous  system  of  the  jellyfish  in  which  the 
conductive  tissue  forms  a  ring  about  the  periphery  of  the  animal,  separated 
from  the  surface  epithelium  and  contractive  tissue  but  connected  to  both  and 
to  different  portions  of  itself  by  its  own  fiber  like  processes.  (Page  8.) 

The  centrally  placed  nervous  system  of  the  worm,  and  of  the  still  more 
advanced  crayfish:  in  both  the  cells  of  the  conductive  tissue  are  centrally 
placed,  thus  facilitating  communication  between  different  portions  of  itself 
and  occupying  the  most  efficient  position  for  rapid  communication  with  any 
portion  of  the  periphery.  In  the  more  advanced  crayfish  there  is  a  special 
development  of  the  fore  part  of  the  central  chain  of  nerve  tissue,  thus  facili- 
tating a  quick  appreciation  of  changes  of  the  environment  in  the  direction 
in  which  the  animal  moves.  (Page  12.} 

Page  14 

Q.  How  is  the  nervous  system  of  mammals  developed  from  the  cells  of 
the  embryo? 

A.  By  the  infolding  of  the  epiblast,  corresponding  to  the  dorsum  of 
that  group  of  cells  of  embryo  from  which  all  this  tissue  of  the  foetus  are 
developed,  there  is  formed  the  neural  canal,  and  on  each  side  a  depressed  cord 
of  cells.  By  a  differentiation  of  the  cells  lining  the  canal  and  forming  the 
cord  the  primitive  spongioblasts  and  neuroblasts  are  formed.  Both  these 
develop  processes.  The  spongioblasts  with  their  processes  form  the  neuroglia 
or  supporting  tissue  of  the  nervous  system.  The  neuroblasts  of  the  neural 
canal  form  nerve  cells  and  their  processes  the  motor  nerves.  These  grow  out 
into  the  body  of  the  embryo  and  form  connections  with  every  active  tissue. 
The  neuroblasts  of  the  lateral  cords  of  cells  develop  into  the  nerve  cells  of  the 
sensory  ganglia.  They  develop  two  processes,  one  forming  peripheral  connec- 
tions with  the  various  specialized  sensitive  cells  of  the  body,  and  the  other 
growing  centrally  among  the  cells  developing  from  the  neural  canal  to  partici- 
pate in  the  formation  of  central  synapses. 

418 


THE  NERVOUS  SYSTEM 

Page  34 

Q.  Describe  a  neuron. 

A.  A  neuron  consists  of  a  nerve  cell  and  its  processes.     A  nerve  cell  pos- 
sesses the  following  parts:     See  text. 

A  nerve  has  the  following  structure:     See  text. 

Page  54 

Q.  Describe  the  different  peripheral  endings  of  nerves. 

A.  See  text,  and  divide  into  sensory  and  motor  nerve  endings. 


Page 


Q.  Classify  nerves. 
A.  See  text. 


Page  74 

Q.  Describe  the  method  of  measuring  the  velocity  of  nerve  impulses  for 
both  motor  and  sensory  nerves. 
A.  See  text. 

Q.  In  what  direction  does  a  nerve  impulse  travel? 
A.  In  both  directions  from  the  stimulated  point. 

Page  76 

Q.  Is  there  any  expenditure  of  energy  caused  by  the  passage  of  a  nerve 
impulse,  how  much  and  how  is  it  estimated? 

A.  A  very  small  amount,  not  enough  to  be  indicated  by  its  transformation 
into  heat,  but  only  by  the  consumption  of  oxygen. 

Page  78 

Q.  What  is  the  demarcation  current? 

A.  The  current  excited  in  a  nerve  by  the  degenerating  changes  following 
injury  to  the  nerve. 

Q.  What  is  the  current  of  action? 

A.  The  current  which  always  accompanies  the  passage  of  a  nerve  impulse. 

Q.  In  a  muscle  nerve  preparation  in  what  order  do  the  tissues  become 
fatigued? 

A.  Motor  end  plate,  muscle.    The  nerve  is  not  known  to  become  fatigued. 

Page  84 

Q.  What  is  summation? 

A.  The  reaction  evoked  by  the  combined  effect  of  several  sub-minimal 
stimuli  following  each  other  at  the  proper  favorable  interval. 

Q.  What  is  the  refractory  period? 

A.  The  period  following  an  excitation  during  which  the  nerve  remains 
incapable  of  response  to  a  second  stimulus. 

420 


THE  NERVOUS  SYSTEM 

Page  86 

Q.  At  what  electrode  does  excitation  of  a  nerve  by  an  electrical  current 
take  place? 

A.  At  the  cathode  at  the  make,  and  anode  at  the  break. 

Q.  What  changes  in  degrees  of  excitability  do  these  special  sites  of  exci- 
tation indicate  and  what  names  are  made  use  of  to  express  such  changes? 

A.  They  indicate  changes  in  excitability  which  are  proportional  to  the 
response  evoked,  that  change  occurring  at  the  cathode  being  named  cathelec- 
trotones,  and  at  the  anode,  anelectrotones,  so  that  the  development  of  the  one 
and  passing  off  of  the  other  is  what  causes  excitation. 

Page  90 

Q.  How  much  of  the  nerve  may  be  involved  in  anelectrotones  or  cath- 
electrotones? 

A.  The  greater  the  strength  of  the  current  the  greater  the  length  of  the 
nerve  which  is  in  anelectrotonus,  the  remainder  of  the  nerve  being  in  catelec- 
trotonus. 

Q.  What  effect  do  the  facts  expressed  in  the  last  answer  have  upon  the 
passage  of  the  nerve  impulse  and  what  name  is  given  to  the  phenomenon? 

A.  The  nerve  impulse  may  be  blocked  at  the  anode  by  a  high  degree  of 
anelectrotones  or  at  the  cathode  by  a  swing  back  from  a  very  high  state  of 
cathelectrotones  to  a  very  low  state  of  cathelectrotones.  The  phenomena 
result  in  a  response  to  stimulation  which  is  different  for  different  strengths 
of  the  current  used,  and  this  fact  is  called  Pfluger  's  law. 

Page  94 

Q.  What  is  the  order  of  strength  of  contraction  in  the  human  being  when 
the  electrode  must  be  applied  on  the  surface  of  the  skin  at  a  distance  from 
the  nerve,  and  why  is  this  order  different  from  that  order  to  be  expected  when 
the  electrodes  are  applied  directly  to  the  nerves  according  to  Pfliiger 's  law? 
A.  1.  See  text  for  order. 

2.  Because  there  is  a  greater  strength  of  current,  due  to  convergence 
of  the  lines  of  force  between  the  electrodes,  in  that  portion  of 
the  nerve  which  is  nearest  to-  the  stimulating  electrode. 

Page  96 

Q.  What  is  the  current  of  polarization  and  to  what  is  it  due? 

A.  The  current  of  polarization  is  a  current  independent  of  vital  changes 
occurring  in  an  electrically  stimulated  nerve,  and  is  due  to  the  difference  in 
potential  which  depends  upon  the  collection  of  ions  upon  the  electrodes  and 
bearing  an  opposite  charge  to  the  electrodes.  These  ions  arise  in  the  elec- 
trolyte of  the  nerve  sheath,  and  the  phenomenon  is  common  to  any  electrolyte 
carrying  a  current. 

Page  100 

Q.  What  are  the  conditions  affecting  the  excitatory  effect  in  an  electri- 
cally stimulated  nerve? 

A.  1.  The  rate  of  change  in  the  make  or  break.     There  is  an  optional 

rate  of  change. 
2.  The  intensity  of  the  current.     There  is  an  optional  intensity. 

422 


THE  NERVOUS  SYSTEM 

3.  The  duration  of  the  current.     There  is  an  optional  duration.     This 

duration  is  different  for  nerve,  motor  end  plate  and  muscle. 

4.  The  temperature.     Warming  the  nerve  of  mammal  increases  its  irri- 

tability. 

Page  104 

Q.  In  what  direction  may  an  impulse  pass  across  a  motor  end  plate? 
A.  As  is  the  case  in  all  synapses,  only  in  the  normal  direction. 

Page  110 

Q.  Describe  the  gross  anatomy  of  the  spinal  cord. 
A.  See  text. 

Page  116 

Q.  What  are  the  groups  of  nerve  cells  in  the  gray  matter? 
A.  1.  Anterior  horn  cells.     The  motor  cells. 

2.  Small  cells  in  the  lateral  portion  of  the  base  of  the  anterior  horn, 

the  motor  cells  of  the  sympathetic  nerves. 

3.  Cells   in    the    lateral   portion   of    the    base    of    the   posterior   horn, 

Clarke's  column,  the  axons  of  which  form  the  dorso-lateral  cere- 
bellar  tract. 

4.  Cells  of  the  posterior  horn,   many  of  which  are  receiving  cells  of 

fibers  of  the  posterior  nerve  roots,  and  others  association  cells. 

Page  120 

Q.  What  are  some  of  the  methods  of  tracing  the  systems  of  neurons? 
A.  The  Myelination  Method.    See  text  for  explanation. 
The  Wallerian  Method.    See  text  for  explanation. 

Page  126 

Q.  What  is  the  termination  of  the  fibers  of  the  posterior  nerve  roots? 
A.  There  are  5  sets  of  fibers:  those  forming 

1.  Lissauer's  column. 

2.  The  columns  of  Goll  and  Burdach. 

3.  The  fibers  ending  in  the  cells  of   the  posterior  horn,   from  which 

impulses  are  carried  onward  to  the  anterior  lateral  column  of 
the  opposite  side,  to  the  anterior  horn  of  same  side,  to  posterior 
horn  of  opposite  side,  to  Clarke's  columns  of  cells,  to  small 
cells  of  lateral  horn. 

Page  136 

Q.  What  are  the  descending  spinal  tracts? 
A.  See  text. 

Page  140 

Q.  What  are  the  ascending  spinal  tracts? 
A.  See  text. 

Page  142 

Q.  What  sensory  impulses  are  carried  by  the  various  ascending  tracts? 
A.  See  text. 

424 


THE  NERVOUS  SYSTEM 

Page  144 

Q.  What  are  the  symptoms  of  unilateral  section  of  the  spinal  cord  I 
A.  See  text. 

Q.  How  may  the  spinal  functions  be  studied  to  the  best  advantage  and 
why! 

A.  By  dividing  the  cord  from  the  brain,  because  the  functions  of  the 
cord  will  thus  be  undisturbed  by  impulses  from  the  brain. 

Page  146 

Q.  What  condition  is  induced  by  separation  of  the  cord  from  the  brain, 
and  what  are  the  symptoms? 
A.  1.  Spinal  shock. 

2.  Permanent  loss  of  sensation  and  of  voluntary  motion  below  level 

of  lesion. 

3.  Temporary  loss  of  muscular  tone,  of  vascular  tone  and   of  reflex 

response. 

Q.  To  what  are  the  symptoms  of  spinal  shock  due? 

A.  The  permanent  symptoms  are  due  to  division  of  the  paths  of  sensory 
perception  and  voluntary  motor  impulses.  The  temporary  symptoms  are  due 
to  the  division  of  paths  through  which,  under  normal  conditions,  impulses 
responsible  for  both  vascular  and  skeletal  tone  are  constantly  passing.  These 
paths  include  in  part  ascending  tracts. 

Page  150 

Q.  Define  reflex  action,  explain  its  mechanism,  and  illustrate  by  spinal 
reflexes. 

A.  A  reflex  action  is  any  motor  response  produced  by  a  sensory  stimulus. 
It  involves  an  afferent  limb,  or  sensory  neuron,  conveying  the  sensory  stimulus 
to  the  central  nervous  system,  one  or  more  central  synapses,  across  which  the 
sensory  stimulus  is  transmitted  to  the  motor  or  efferent  neuron.  It  is  illus- 
trated by  the  scratch  reflex,  sole  reflex,  vascular  reflex,  bladder  and  rectal 
reflexes.  The  reflexes  upon  which  muscular  tone  depends  and  tendon  reflexes. 
See  text  for  description. 

Page  156 

Q.  What  are  the  characteristics  of  spinal  reflexes? 
A.  Purpose  like,  etc.     See  text. 

Page  178 

Q.  Define  and  describe  a  synapse  and  what  is  its  function? 

A.  A  synapse  is  the  interval  between  the  terminal  arborizations  of  a 
nerve  fiber  around  the  cell  of  another  neuron  with  which  it  is  functionally 
related.  This  interval  is  not  bridged  by  nerve  fibrillse,  so  that  there  is  no 
direct  continuation  of  nerve  substance  between  one  neuron  and  the  next  one 
in  functional  association  with  it.  The  interval  is  filled  with  a  granular 
material  which  permits  of  the  passage  of  nerve  impulses  in  only  one  direction. 

426 


THE  NERVOUS  SYSTEM 

Page  176 

Q.  What  is  meant  by  the  trophic  functions  of  the  spinal  cord? 

A.  The  spinal  cord  is  constantly  supplying  to  the  peripheral  tissues 
through  special  nerve  impulses,  named  trophic,  which  improve  the  nutrition 
of  these  tissues. 

Page  178 

Q.  How  must  the  brain  be  considered  phylogenetically  and  from  what 
embryological  units  does  it  develop? 

A.  As  modified  anterior  segments  of  the  primitive  cerebro-spinal  axis 
or  canal.  The  four  divisions,  into  which  the  adult  brain  may  be  grossly 
divided,  develop  from  three  primitive  cerebral  cavities  at  the  anterior  end  of 
the  primitive  neural  tube.  These  three  vesicles  are  named  anterior,  middle 
and  posterior  cerebral  vesicles.  The  anterior  vesicle  develops  into  the  lateral 
ventricles  and  cerebral  cortex  and  third  ventricles  of  the  thalami.  The  middle 
vesicle  into  the  aqueduct  of  Sylvius  and  the  brain  stem  with  its  nuclei.  The 
posterior  vesicle  into  the  fourth  ventricle,  the  pons,  cerebellum  and  bulb. 

Page  180 

Q.  How  are  the  retina  of  the  eyes,  the  optic  nerves,  the  olfactory  bulbs 
and  olfactory  nerves  developed? 

A.  By  tubular  protrusions  from  the  anterior  cerebral  vesicles. 

Page  182 

Q.  Describe  the  floor  of  the   fourth  ventricle? 
A.  See  text. 

Page  186 

Q.  Describe  the  third  brain. 

A.  See  text.  Mention  iter  of  Sylvius  and  corpora  quadrigemina  and 
geniculate  bodies  and  their  connections. 

Q.  Describe  the  third  ventricle. 

A.  See  text.  Mention  its  shape,  its  roof,  the  corpus  callosum  and  fornix, 
its  lateral  walls,  the  optic  thalami,  its  three  commissures,  and  in  the  floor 
the  optic  chiasma,  the  pituitary  body  and  at  its  posterior  corner  the  pineal 
gland. 

Page  192 

Q.  Describe  the  lateral  ventricles. 

A.  See  text.  Mention  the  body  and  three  horns.  The  roof  is  formed  by 
the  corpus  callosum;  the  floor  of  the  body  and  roof  of  the  inferior  horn  by  the 
optic  thalami,  the  stria  semicircularis  and  caudate  nucleus  with  its  tail,  the 
internal  wall  by  the  septum  lucidum  (anterior  horn),  the  fornix  and  the  choroid 
plexus  (body) ;  and  from  above  down,  the  forceps  major  and  hippocampus 
minor  or  calcar  avis  (the  posterior  horn),  the  choroid  plexus  and  hippo- 
campus major  (inferior  horn).  The  external  wall  of  all  horns  and  body  by 
the  cerebral  convolutions.  The  lateral  ventricles  communicate  with  the  third 
ventricles  by  the  foramen  of  Monro,  which  opens  into  the  anterior  end  of 
the  third  ventricle  beneath  and  behind  the  pillar  of  the  fornix,  from  the 
juncture  of  the  anterior  horn  and  body  of  the  lateral  ventricle.  It  is  the 

428 


THE  NERVOUS  SYSTEM 

remnant  of  the  neck  of  the  bud  from  the  primitive  anterior  cerebral  vesicle, 
the  cavity  of  which  forms  the  lateral  ventricles  and  the  eye  walls. 

Page  204 

Q.  Describe  the  cerebral  hemispheres. 

A.  The  cerebral  hemispheres  are  divided  into  five  lobes  by  four  important 
fissures.  The  fissure  of  Eolando  (see  text  for  position)  separates  the  frontal 
lobe  on  the  external  surface  of  brain  from  the  parietal  lobe.  The  fissure 
of  Sylvius  (see  text  for  position)  forms  the  lower  boundary  of  the  frontal 
lobe  and  parietal  lobe  on  the  external  surface.,  separating  them  from  the 
temporal  lobes.  The  parieto -occipital  fissure  (see  text  for  position),  which 
separates  the  occipital  lobe  from  the  parietal  and  limbic  lobes  on  the  internal 
surface  of  the  hemispheres,  and  indicates  the  separation  of  the  occipital  from 
the  parietal  and  temporal  lobes  on  the  external  surface  of  the  hemispheres. 

Page  234 

Q.  Describe  the  internal  structure  of  the  medulla. 

A.  The  internal  structure  of  the  meHulla  differs  from  that  of  the  spinal 
cord  as  a  result  of  the  opening  out  of  the  central  canal  of  the  cord  into  the 
fourth  ventricle  of  the  bulb.  The  disposition  of  the  gray  matter  represents 
a  displacement  of  the  gray  matter  of  the  cord  in  a  posterior  and  then  a 
lateral  direction  to  a  position  lateral  in  the  floor  of  the  ventricle.  In  this 
position  it  forms  the  nuclei  of  the  cranial  nerves  in  the  positions  described 
and  illustrated  in  the  text. 

A  second  difference  between  the  internal  structure  of  the  medulla  and 
the  cord  is  due  to  the  passage  of  the  fibers  of  the  pyramidal  tract  from  the 
decussation  on  the  front  to  the  posterolateral  position  which  they  occupy  in 
the  cord.  In  this  passage  they  amputate  and  break  up  the  gray  matter  of 
the  anterior  horns,  forming  the  lateral  nucleus. 

A  third  difference  is  due  to  the  development  of  new  gray  matter,  the 
nucleus  gracilis  and  cuneatus,  in  the  posterolateral  regions  of  the  bulb  in 
which  the  columns  of  Goll  and  Burdach  end.  Another  important  mass  of 
gray  matter  appearing  in  the  upper  part  of  the  bulb  is  the  olivary  nucleus. 
On  section  it  appears  scalloped  shaped,  with  its  concavity  directed  toward  the 
center  of  the  bulb.  Its  efferent  fibers  are  afferent  to  the  cortex  of  the  cere- 
bellum. 

Page  258 

Q.  Describe  the  cerebellum. 

A.  The  cerebellum  is  an  isolated  mass  of  brain  tissue  about  2%"  x  1*4", 
situated  in  the  posterior  fossa  of  the  cranium  and  composed  of  two  lateral 
lobes  and  a  central  mass,  forming  a  rounded  intervening  eminence  above  and 
below.  The  surface  of  all  these  three  lobes  is  thrown  into  convolutions  and 
contains  immediately  beneath  it  the  gray  matter  of  the  cerebellum.  The 
tissue  beneath  this  surface  layer  consists  of  white  fibers  which  enter  for 
the  most  part  the  cerebellum  in  the  three  peduncles  and  pass  to  the  cells  in 
the  gray  matter.  Within  the  center  of  the  cerebellum  are  four  nuclei  (see 
text),  which  receive  the  efferent  fibers  from  the  gray  matter  of  the  cortex  and 

430 


THE  NERVOUS  SYSTEM 

send  efferent  fibers  from  the  cerebellum  to  the  nuclei  pontis  and  red  nucleus. 
While  many  fibers  passing  to  the  cerebellum  probably  make  connections  with 
the  deep  nuclei,  particularly  those  from  the  vestibular  and  Deiters'  nuclei, 
yet  in  general  the  afferent  fibers  to  the  cerebellum  pass  to  the  cortex  and 
the  efferent  fibers  pass  out  from  its  deep  nuclei.  The  afferent  fibers  to  the 
cerebellum  pass  to  it  through  the  three  peduncles  (see  text,  page  298). 

Page  260 

Q.  Describe  the  third  brain. 

A.  Above  the  pons  the  central  canal  of  the  cerebro-spinal  system  becomes 
again  a  closed  narrow  canal,  until  it  opens  into  the  third  ventricle.  The 
gray  matter  immediately  surrounding  it  constitutes  the  nuclei  of  the  oculo- 
motor nerves.  The  superior  peduncles  of  the  cerebellum  converge  below  the 
canal  to  cross  in  a  decussation  and  end  in  the  cells  of  red  nucleus,  a  large 
mass  of  gray  matter  situated  in  the  substance  of  the  third  brain  below  the 
forepart  of  the  aqueductus  Sylvii.  Above  the  forepart  of  the  aqueduct,  form- 
ing rounded  eminences  on  the  dorsal  surface  of  the  third  brain,  are  two 
masses  of  gray  matter  on  each  side  of  the  middle  line,  the  superior  and  inferior 
corpora  quadrigemina.  The  superior  corpora  quadrigemina  receive  fibers  from 
the  optic  nerve  and  cerebellum.  The  inferior  corpora  quadrigemina  receives 
the  lateral  fillet  and  is  related  in  function  to  the  sense  of  hearing. 

Page  262 

Q.  What  is  the  posterior  longitudinal  bundle  and  its  function? 

A.  A  longitudinal  bundle  of  nerve  fibers  is  seen  in  all  sections  of  the  third 
brain  just  ventral  to  the  Sylvian  aqueduct  and  is  continued  downwards  through 
the  pons  and  medulla,  being  continuous  with  the  tract  of  Marie  or  the 
anterolateral  association  tracts  of  the  spinal  cord.  By  means  of  this  tract 
a  connection  is  established  between  all  the  nuclei  of  the  cranial  nerve.  See 
Fig.  126. 

Page  304 

Q.  Describe  the  subcortical  masses  of  gray  matter,  and  the  external  and 
internal  capsule. 

A.  The  subcortical  nuclei,  apart  from  those  belonging  to  the  third  brain, 
are  the  claustrum,  the  lenticular  nucleus,  the  caudate  nucleus  and  the  optic 
thalami. 

The  claustrum  is  a  thin  mass  of  gray  matter  immediately  underlying 
the  Island  of  Keil,  being  separated  from  it  by  a  thin  layer  of  white  matter. 

The  lenticular  nucleus,  consisting  of  the  putamen  and  the  globus  pallidus, 
is  a  wedged  shaped  (on  coronal  section)  mass  of  gray  matter  immediately 
internal  to  the  claustrum  and  separated  from  it  by  a  thin  layer  of  white 
matter,  the  external  capsule. 

The  caudate  nucleus  is  a  large  mass  of  gray  matter  consisting  of  a 
rounded  anterior  head  and  a  long  tail  tapering  out  posteriorly,  the  whole 
body  being  shaped  somewhat  like  a  long  turnip  or  drawn-out  pear.  It 
curves  around  the  external  periphery  of  the  optic  thalamus,  forming  the  ex- 
ternal part  of  the  floor  of  the  body  of  the  lateral  ventricle  and  the  roof  of 
the  external  part  of  the  inferior  horn.  It  and  the  optic  thalamus,  which  it,  in 

432 


THE  NERVOUS  SYSTEM 

part,  encircles,  is  separated  from  the  lenticular  nucleus  by  an  important  mass 
of  gray  matter,  the  internal  capsule. 

The  optic  thalamus  itself  is  a  large  mass  of  gray  matter  forming  the 
external  wall  of  the  third  ventricle  and  the  floor  of  the  lateral  ventricle  and 
bounded  externally  by  the  internal  capsule  and  in  part  by  the  caudate  nucleus. 

Page  SOS 

Q.  Describe  the  internal  capsule,  the  pyramidal  tracts  and  the  cerebro- 
pontine  tracts. 

A.  The  internal  capsule  is  a  thick  stratum  of  white  fibers  passing  in  a 
general  vertical  direction  between  the  cerebral  cortex  and  the  pons.  It  is 
flanked  by  the  optic  thalamus  internally  and  the  lenticular  nucleus  externally. 
Its  fibers  pass  to  and  from  all  parts  of  the  cerebral  cortex.  Below  they  con- 
stitute the  crura  cerebri,  forming  a  great  thick  bundle  on  each  side  of  the 
middle  line  ventral  to  the  third  brain,  and  converging  to  plunge  into  the 
upper  part  of  the  pons.  See  Figs.  100,  135.  Through  it  run  all  the  sensory 
tracts  from  the  optic  thalami,  continuing  onward  the  mesial  fillet  to  the 
cerebral  cortex;  also  within  it  pass  the  large  motor  tracts  made  of  fibers 
which  are  the  axis  cylinders  of  the  cells  of  the  motor  area  of  the  cortex. 
These  pass  down  through  the  central  regions  of  the  internal  capsule  and 
crura  cerebri  to  the  pons.  They  plunge  through  the  anterior  portion  of  the 
substance  of  the  pons  and  appear  on  each  side  of  the  middle  line  of  the 
anterior  surface  of  the  medulla,  where  they  constitute  the  two  rounded  emi- 
nences known  as  the  pyramids.  Immediately  below  the  pyramids  they  decus- 
sate and  pass  downward  in  the  lateral  columns  of  the  cord  as  the  pyramidal 
tracts,  to  terminate  at  various  levels  of  the  cord,  either  directly  or  indirectly 
around  the  anterior  horn  cells.  The  internal  capsule  also  contains  fronto- 
pontine  and  temporopontine  fibers,  passing  from  the  frontal  and  temporal 
lobes  through  the  anterior  and  posterior  limbs  respectively  of  the  internal 
capsule  and  the  mesial  and  external  portions  respectively  of  the  crura  to  the 
cells  of  the  formatio  reticularis. 

Page  SS6 

Q.  What  are  the  portions  of  the  olfactory  mechanism  in  their  physiological 
order? 

A.  1.  The  peripheral  bipolar  cells  in  the  nasal  mucosa. 

2.  The  arborizing  connection  between  the  central  process  of  these  cells 

and  the  peripheral  processes  of  the  mitral  cells. 

3.  The  mitral  cells  in  the  olfactory  bulbs. 

4.  The  olfactory  tracts. 

5.  The  portions  of  the  brain  and  their  connecting  tracts  which  form 

the  olfactory  mechanism.    See  text. 

Page  340 

Q.  Describe  the  optic  nerves? 

A.  The  optic  nerves  are  two  large  bundle  of  nerve  fibers,  these  fibers 
being  axons  of  the  ganglion  cells  in  the  anterior  layers  of  the  retinae,  which  pass 
through  the  optic  foramen  to  the  optic  groove  on  the  upper  surface  of  the 
body  of  the  sphenoid.  In  this  groove  the  fibers  from  the  inner  half  of  each 

434 


THE  NERVOUS  SYSTEM 

retina  decussate  and  pass  them  with  the  fibers  from  the  external  half  of 
each  retina  in  two  large  bundles,  the  optic  tracts,  around  the  crura  cerebri 
(see  Fig.  100)  to  the  external  geniculate  body,  the  superior  corpora  quad- 
rigemina  and  the  posterior  portion  of  the  optic  thalami.  Axons  of  cells  in 
these  nuclei  then  continue  the  visual  sensations  through  the  posterior  portion 
of  the  internal  capsule,  the  optic  radiations,  to  the  occipital  lobes. 

Page  SU 

Q.  What  are  the  oculo-motor  nerves  and  their  function! 

A.  The  oculo-motor  nerves  are  the  third,  fourth  and  sixth.  The  nuclei 
of  origen  of  the  third  and  fourth  surround  the  Sylvian  aqueduct.  That  of 
the  sixth  is  beneath  the  floor  of  the  pontine  portion  of  the  medulla.  They 
supply  oculo-motor  impulses  to  the  recti  muscles  of  the  eyeball — the  sixth 
supplying  the  external  rectus,  the  fourth  the  superior  oblique  and  the  third 
the  other  muscles  and  sphincter  pupili  and  ciliary  muscle. 

Q.  What  is  the  function  of  the  fifth  nerve? 
A.  See  text. 

Q.  What  is  the  function  of  the  seventh  nerve? 
A.  See  text. 

Page  346 

Q.  What  is  the  function  of  the  eighth  nerve,  and  the  central  connections 
of  its  fibers? 

A.  The  eight  nerve  supplies  to  the  central  nervous  system  two  sets  of 
impulses  through  the  two  separate  portions  of  which  it  consists. 

1.  The  vestibular  portion  is  composed  of  axons  of  bipolar  nerve  cells 

which  have  retained  their  original  bipolar  morphology,  and  the 
peripheral  processes  of  which  end  in  the  saccule,  vestibule  and 
semicircular  canals.  It  therefore  transmits  sensations  of  equilib- 
rium. The  central  processes  end  in  the  vestibular  nucleus  be- 
neath the  mid-lateral  portion  of  the  floor  of  the  fourth  ven- 
tricle. Its  axons  form  important  connections  with  Dieters' 
and  Bechterew's  nuclei,  two  very  important  nuclei  in  the  same 
region.  From  these  nuclei  fibers  pass  to  the  roof  nuclei  and 
cortex  of  the  cerebellum.  Doubtless  some  fibers  of  the  vestibular 
nucleus  pass  directly  to  the  roof  nuclei  of  the  cerebellum.  They 
transmit  impulses  excited  by  changes  in  the  position  of  the  body 
as  a  whole. 

2.  The  auditory  portion  of  the  eighth  nerve  arises  in  the  bipolar  cells 

situated  in  the  crest  of  the  cochlea.  These  also  have  retained 
their  embryonic  bipolar  morphology.  Their  peripheral  processes 
terminate  in  the  auditory  epithelium  of  the  canal  of  Corti. 
Their  central  processes  end  in  the  cells  of  the  auditory  tubercle 
at  the  extreme  mid-lateral  angle  of  the  floor  of  the  medulla. 
The  impulses  are  carried  across  the  middle  line  to  the  opposite 
side  of  the  medulla  to  form  the  ascending  tract  of  the  lateral 
fillet  by  two  sets  of  fibers,  one  superficial  on  the  floor  of  the 
medulla,  the  stria  acoustica,  and  the  other  running  directly  to 
the  lateral  fillet  forming  a  decussation  imbedded  deeply  in  the 
medulla  and  known  as  the  trapezium.  By  means  of  the  lateral 
fillet  the  impulses  pass  to  the  internal  geniculate  body  and  the 

436 


THE  NERVOUS  SYSTEM 

inferior  corpora  quadrigemina,  an'd  thence  through  the  internal 
capsule  and  finally  by  the  auditory  radiations  to  the  temporal 
convolutions. 

Page  350 

Q.  What  is  the  function  of  the  ninth,  tenth  and  twelfth  nerves? 
A.  See  text. 

Q.  How  may  the  functions  of  the  brain  be  studied? 
A.  See  text. 

Page  352 

Q.  What  are  the  afferent  impulses  received  by  the  medulla? 
A.  See  text. 

Q.  Describe  the  activities  of  the  bulbo-spinal  animal  and  how  it  differs 
from  the  spinal  animal. 

A.  Its  cardiac,  arterial  and  respiratory  functions  are  normal.  There  is 
a  little  greater  stability  of  the  spinal  reflexes,  due  to  the  preservation  of  a 
little  greater  degree  of  muscular  tone. 

Page  354 

Q.  Describe  the  activities  of  the  pontine-bulbo-spinal  animal. 

A.  It  shows*  all  the  reactions  of  the  bulbo-spinal  animal,  but  its  position 
and  movements  will  preserve  the  normal  position  of  its  center  of  gravity. 
There  is  greater  increase  in  the  stability  of  reflex  movement.  If  such  an 
animal  loses  its  cerebellum  it  becomes  spontaneously  active. 

Q.  Describe  the  activities  of  the  midbrain-bulbo-spinal  animal. 

A.  The  mammal  exhibits  decerebrate  rigidity  (see  text  for  definition) 
and  of  course  all  the  activities  of  which  the  pontine-bulbo-spinal  animal  is 
capable. 

The  frog  exhibits  little  that  is  abnormal  in  its  deportment. 

Page  356 

Q.  Describe  the  deportment  of  the  thalamo-spinal  animal. 

A.  Its  deportment  exhibits  so  little  that  is  abnormal  that  only  unusual 
tests  designed  to  bring  into  play  activities  which  involve  the  exercise  of 
memory,  and  therefore  choice,  fear,  affection,  etc.,  are  capable  of  detecting 
anything  abnormal. 

Q.  What  are  the  functions  of  the  cerebellum? 

A.  The  cerebellum  is  the  receiving  station  for  the  important  nerves  of 
both  proprioceptive  and  exteroceptive  impulses  of  static  sensation.  The  end- 
ings of  these  nerves  are  so  associated  with  association  neurons  of  the  cerebrum, 
including  the  efferent  control  of  the  cerebrum  over  the  spinal  functions, 
although  there  is  probably  some  more  direct  efferent  relation  between  the 
efferent  cerebellar  impulses  and  the  spinal  functions  through  the  vestibulo- 
spinal  tract,  that  there  constantly  leave  the  cerebellum  a  flow  of  efferent 
impulses  which  provide  for  the  most  efficient  co-ordination  of  individual  muscles 
during  either  states  of  rest  or  activity;  and  also,  through  chiefly  the  extero- 
ception  impulses  of  static  sensation,  the  maintaining  of  the  best  balanced  posi- 
tion of  the  center  of  gravity  of  the  whole  body. 

438 


THE  NERVOUS  SYSTEM 

Page  860 

Q.  Describe  the  histology  of  the  cerebellar  cortex. 

A.  The  characteristic  cell  of  the  cerebellum  is  the  cell  of  Purkinje.  It 
is  flask  shaped  and  from  its  apex  a  rich  turf  of  dendrites  arise.  Its  axon 
arises  from  the  base  of  the  cell  and  passes  into  the  white  substance  of  the 
cerebellum  to  the  central  nuclei.  This  cell  lies  between  two  layers  of  smaller 
cells,  the  dendrites  and  axons  of  which  are  chiefly  associative  in  function. 

Page  362 

Q.  What  are  the  symptoms  of  the  cerebellar  less  animal? 
A.  Asthenia,  atonia  and  astasia.    See  text  for  definition. 

Page  868 

Q.  What  is  the  characteristic  of  cerebellar  ataxia,  and  how  does  it  differ 
from  the  two  types  of  spinal  ataxia? 

A.  A  failure  to  maintain  the  normal  position  of  the  center  of  gravity 
of  the  body.  It  might  be  described  as  a  top-heavy  ataxia,  exactly  analogous 
to  the  ataxia  of  a  drunken  individual.  It  differs  from  the  spinal  ataxia  of 
lateral  sclerosis,  in  which  the  pyramidal  tracts  are  degenerated,  in  that  in 
the  latter  all  movements  are  exaggerated;  the  ataxia  is  due  to  over- movement. 
It  differs  from  spinal  ataxia  of  tabes  dorsalis,  in  which  the  posterior  columns 
of  Goll  and  Burdach  are  degenerated,  in  that  in  tabes  the  ataxia  is  character- 
ized by  an  inexactness  of  all  movements  depending  upon  a  blunting  of  the 
sensations  inf  ormatory  of  the  exact  position  of  individual  muscles  and  tendons 
and  joints. 

Page  370 

Q.  What  additional  possibilities  of  action  does  the  possession  of  the  cere- 
bral hemispheres  afford  an  animal? 

A.  That  alteration  of  activity  which  depends  upon  memory.  In  the 
animal  deprived  of  its  cerebral  hemispheres  the  nervous  path  between  the 
incoming  sensory  impulses  and  action  is  so  direct  that  these  animals  respond 
to  external  stimulation  with  a  machine-like  certainty. 

An  animal  with  a  cerebral  hemisphere  responds  in  an  uncertain  manner, 
because  of  the  influence  of  impulses  along  many  association  tracts  which 
have  been  brought  into  relation  with  incoming  stimuli  by  past  experiences. 
In  virtue  of  these  intervening  impulses  between  sensation  and  action  a  deport- 
ment results  which,  according  to  the  function  of  the  action,  is  classified  as  an 
expression  of  all  the  higher  possibilities  of  which  the  mind  is  capable,  such  as 
love,  fear,  self-restraint,  etc. 

Page  372 

Q.  What  region  of  the  cerebral  cortex  is  the  so  to  speak  terminal  dis- 
charging station  of  motion? 
A.  See  text. 

Page  378 

Q.  What  is  the  distinguishing  characteristic  of  movements  excited  by 
stimulation  of  the  cerebral  cortex? 

A.  Their  similarity  to  the  voluntary  movements  of  the  animal,  involving 
such  a  co-ordination  of  inhibition  and  contraction  that  the  movement  becomes 
purposeful  in  the  highest  sense. 

440 


THE  NERVOUS  SYSTEM 

Page  S80 

Q.  What  is  the  difference  in  the  character  of  the  control  exercised  over 
voluntary  movement  by  the  cerebrum  and  cerebellum? 

A.  The  cerebrum  initiates  motion  and  determines  what  muscles  shall  be 
called  into  play  in  the  accomplishment  of  a  definite  movement  or  combinations 
of  motion,  while  the  cerebellum  controls  the  varying  degree  of  contraction  and 
relaxation  of  muscles  only  so  far  as  is  necessary  for  the  accomplishment  of 
perfect  co-ordination  and  maintenance  of  the  correct  position  of  the  center 
of  gravity  of  the  body  during  the  progression  of  these  movements  or  the  inter- 
vening states  of  muscular  contraction. 

Page  384 

Q.  What  area  of  the  cortex  is  associated  with  the  reception  of  sensations 
of  muscular  sensations  of  touch,  temperature  and  pain? 

A.  Tactile  sensation  of  touch  and  muscular  sensations  are  received  first 
by  the  cells  of  the  ascending  parietal  convolution  immediately  posterior  to 
the  fissure  of  Kolando,  the  inferior  parietal  and  supramarginal  convolutions. 

Sensations  of  pain  are  received  by  cells  fairly  widely  distributed  in  the 
cortex.  The  location  has  not  been  exactly  identified.  Those  cells  receiving 
the  sensations  of  temperature  have  not  been  definitely  located,  but  they  prob- 
ably occupy  areas  common  to  the  cells  receiving  cutaneous  sensibility. 

Page  386 

Q.  What  areas  in  the  cortex  receive  visual  sensations? 
A.  The  cortex  of  the  occipital  lobes. 

Page  388 

Q.  What  area  in  the  cortex  receives  auditory  sensations? 

A.  The  cortex  of  the  superior  temporal-  convolution;  but  there  is  evidence 
that  this  region  is  not  the  only  one  devoted  to  the  reception  of  auditory 
sensations,  and  that  other  more  widely  distributed  areas  also  participate 
in  this  function,  though  their  exact  location  is  as  yet  unidentified. 

Q.  What  areas  in  the  cortex  receive  sensations  of  smell  and  taste? 

A.  Many  portions  of  the  limbic  lobe,  including  particularly  the  inferior 
surface  of  the  frontal  lobe,  the  portion  of  limbic  lobe  contiguous  to  the  corpus 
callosum,  the  uncus  and  the  hippocampus  major. 

Page  390 

Q.  What  is  the  function  of  the  large  areas  of  the  cortex  intervening 
between  sensory  and  .motor  areas  described? 

A.  These  so-called  silent  areas  perform  association  functions.  By  them 
the  primary  sensations  are  grouped  into  concepts,  and  the  concepts  themselves 
are  compounded  and  compared  with  other  similar  concepts,  suggested  because 
of  their  analogy  by  them  and  the  cerebral  states  depending  upon  this  variety 
of  concepts,  stimulated  so  that  the  rudiments  of  the  higher  faculties  of  choice 
and  judgment  and  decision  are  possible.  These  highest  faculties  are  performed 
by  the  frontal  portions  of  the  cortex. 

442 


THE  NERVOUS  SYSTEM 

Page  S92 

Q.  How  may  words  be  psychologically  defined,  and  how  do  they  facilitate 
cerebral  processes? 

A.  Words  are  names  given  by  the  mind  to  concepts  of  varying  complexity, 
and  by  the  use  of  these  symbols  for  complex  cerebral  process,  the  cerebral 
states  involved  in  concepts  of  extreme  complexity  may  be  quickly  produced, 
and  thus  intricate  thinking  facilitated  or  made  possible. 
• 

Page  394 

Q.  What  is  aphasia,  how  many  kinds  of  aphasia  are  there,  and  to  what 
are  they  due? 

A.  Aphasia  is  an  impairment  in  the  power  of  speech.  It  may  be  an 
anarthria,  due  to  a  pure  inability  to  phonate. 

It  may  be  a  motor  aphasia,  due  to  an  inability  to  associate  or  select 
the  proper  words  to  express  properly  formed  concepts. 

It  may  be  sensory,  due  to  the  inability  to  associate  with  the  name  of  a 
concept  its  proper  sound  as  heard  or  form  as  written. 

Page  400 

Q.  Where  is  the  thermotaxic  center  of  the  body  situated? 
A.  Probably  in  the  corpus  striatum.     See  text. 

Q.  Describe  the  histology  of  the  cerebral  cortex? 

A.  The  pyramidal  shaped  cell,  with  apical  and  lateral  dendrites  and  basal 
axon,  is  the  typical  cerebral  cell.  It  is  disposed  in  several  layers  composed 
of  pyramidal  cells  of  different  size,  and  is  entirely  associative  in  function. 
In  addition  to  these  cells  there  are  other  layers  of  differently  shaped  cells, 
superficial  and  deeper  to  them,  and  all  layers  are  separated  or  crossed  by 
strata  of  fibers.  The  thickness  of  both  layers  of  cells  and  of  fibers  differs 
according  to  the  functions  of  different  portions  of  the  cerebral  cortex. 

Page  406 

Q.  What  is  the  vegetative  nervous  system,  and  into  how  many  portions  is 
it  divided? 

A.  In  general  that  system  the  nerves  of  which  supply  the  involuntary 
muscles.  It  is  divided  into  the  cranial  or  autonomic,  and  the  spinal  or 
sympathetic  portions. 

Q.  What  cranial  nerves  contain  fibers  of  the  vegetative  nervous  system, 
and  what  do  these  nerves  supply? 

A.  The  third  cranial,  the  vegetative  nerves  of  which  supply  the  sphincter 
pupili  and  the  ciliary  muscle. 

The  seventh  nerve,  the  vegetative  nerves  of  which  supply  the  sublingual 
and  submaxillary  glands  with  secretory  and  vaso-dilator  nerves  to  the  parotid 
gland. 

The  tenth  is  entirely  vegetative.  It  supplies  motor  impulses  to  the 
alimentary  tract  as  far  as  the  ileocolical  valve,  inhibitory  impulses  to  the 
heart,  motor  impulses  to  the  bronchi,  and  secretory  fibers  to  the  stomach 
and  pancreas.  It  contains  afferent  fibers,  passing  to  the  important  medullary 

444 


THE  NERVOUS  SYSTEM 

centers,  and  through  which  the  heart  beat  is  slowed  and  respiration  is  quick- 
ened and  the  blood  pressure  lowered. 

Q.  Describe  the  anatomy  and  ganglia  of  the  spinal  sympathetic  nerves. 
A.  See  text. 

Page  414 

Q.  What  are  the  connections  and  functions  of  the  superior  cervical  sym- 
pathetic ganglia? 
A.  See  text. 

Q.  What  are  the  functions  of  the  first  five  dorsal  nerves  making  connec- 
tions with  the  cervical  and  stellate  ganglia? 
A.  See  text.  • 

Q.  What  sympathetic  nerves  supply  the  upper  limbs  and  what  are  their 
functions? 

A.  See  text. 

Q.  What  sympathetic  nerves  supply  the  lower  limbs,  and  what  is  their 
function? 

A.  See  text 

Q.  What  is  the  nerve  supply  of  the  pelvic  plexus,  and  the  function  ful- 
filled by  them? 
A.  See  text. 

Q.  What  nerves  form  the  nervi  erigentes,  and  what  is  their  function? 
A.  See  text. 


PAUL  B.  HOEBER,  67-69  EAST  59TH  STREET,  NEW  YORK 

446 


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